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For centuries, people have tried to understand the behaviors and beliefs associated with falling in love. What explains the wide range of emotions people experience? How have notions of romance evolved over time? As digital media becomes a permanent fixture in people’s lives, how have these technologies changed how people meet?

Examining some of these questions are Stanford scholars.

From the historians who traced today’s ideas of romance to ancient Greek philosophy and Arab lyric poetry, to the social scientists who have examined the consequences of finding love through an algorithm, to the scientists who study the love hormone oxytocin, here is what their research reveals about matters of the heart.

The evolution of romance

How romantic love is understood today has several historical origins, says Robert Pogue Harrison , the Rosina Pierotti Professor in Italian Literature and a scholar of romance studies.

For example, the idea of finding one’s other half dates back to ancient Greek mythology, Harrison said. According to Aristophanes in Plato’s Symposium , humans were once complete, “sphere-like creatures” until the Greek gods cut them in half. Ever since, individuals have sought after their other half.

Here are some of those origin stories, as well as other historical perspectives on love and romance, including what courtship looked like in medieval Germany and in Victorian England, where humor and innuendo broke through the politics of the times.

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Love in the digital age

Where do people find love today? According to recent research by sociologist Michael Rosenfeld , meeting online is now the most popular way to meet a partner. 

“The rise of the smartphone took internet dating off the desktop and put it in everyone’s pocket, all the time,” said Rosenfeld. He found that 39 percent of heterosexual couples met their significant other online, compared to 22 percent in 2009. 

As people increasingly find connections online, their digital interactions can provide insight into people’s preferences in a partner. 

For example, Neil Malhotra , the Edith M. Cornell Professor of Political Economy, analyzed thousands of interactions from an online dating website and found that people seek partners from their own political party and with similar political interests and ideologies. Here is some of that research. 

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The science of love

It turns out there might be some scientific proof to the claim that love is blind. According to one Stanford study , love can mask feelings of pain in a similar way to painkillers. Research by scientist Sean Mackey found intense love stimulates the same area of the brain that drugs target to reduce pain. 

“When people are in this passionate, all-consuming phase of love, there are significant alterations in their mood that are impacting their experience of pain,” said Mackey , chief of the Division of Pain Medicine. “We’re beginning to tease apart some of these reward systems in the brain and how they influence pain. These are very deep, old systems in our brain that involve dopamine – a primary neurotransmitter that influences mood, reward and motivation.”  

While love can dull some experiences, it can also heighten other feelings such as sociability. Another Stanford study found that oxytocin, also known as the love hormone because of its association with nurturing behavior, can also make people more sociable. Here is some of that research. 

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Sure, your heart thumps, but let’s look at what’s happening physically and psychologically

“They gave each other a smile with a future in it.” — Ring Lardner

Love’s warm squishiness seems a thing far removed from the cold, hard reality of science. Yet the two do meet, whether in lab tests for surging hormones or in austere chambers where MRI scanners noisily thunk and peer into brains that ignite at glimpses of their soulmates.

When it comes to thinking deeply about love, poets, philosophers, and even high school boys gazing dreamily at girls two rows over have a significant head start on science. But the field is gamely racing to catch up.

One database of scientific publications turns up more than 6,600 pages of results in a search for the word “love.” The National Institutes of Health (NIH) is conducting 18 clinical trials on it (though, like love itself, NIH’s “love” can have layered meanings, including as an acronym for a study of Crohn’s disease). Though not normally considered an intestinal ailment, love is often described as an illness, and the smitten as lovesick. Comedian George Burns once described love as something like a backache: “It doesn’t show up on X-rays, but you know it’s there.”

Richard Schwartz , associate professor of psychiatry at Harvard Medical School (HMS) and a consultant to McLean and Massachusetts General (MGH) hospitals, says it’s never been proven that love makes you physically sick, though it does raise levels of cortisol, a stress hormone that has been shown to suppress immune function.

Love also turns on the neurotransmitter dopamine, which is known to stimulate the brain’s pleasure centers. Couple that with a drop in levels of serotonin — which adds a dash of obsession — and you have the crazy, pleasing, stupefied, urgent love of infatuation.

It’s also true, Schwartz said, that like the moon — a trigger of its own legendary form of madness — love has its phases.

“It’s fairly complex, and we only know a little about it,” Schwartz said. “There are different phases and moods of love. The early phase of love is quite different” from later phases.

During the first love-year, serotonin levels gradually return to normal, and the “stupid” and “obsessive” aspects of the condition moderate. That period is followed by increases in the hormone oxytocin, a neurotransmitter associated with a calmer, more mature form of love. The oxytocin helps cement bonds, raise immune function, and begin to confer the health benefits found in married couples, who tend to live longer, have fewer strokes and heart attacks, be less depressed, and have higher survival rates from major surgery and cancer.

Schwartz has built a career around studying the love, hate, indifference, and other emotions that mark our complex relationships. And, though science is learning more in the lab than ever before, he said he still has learned far more counseling couples. His wife and sometime collaborator, Jacqueline Olds , also an associate professor of psychiatry at HMS and a consultant to McLean and MGH, agrees.

Spouses Richard Schwartz and Jacqueline Olds, both associate professors of psychiatry, have collaborated on a book about marriage.

Stephanie Mitchell/Harvard Staff Photographer

More knowledge, but struggling to understand

“I think we know a lot more scientifically about love and the brain than we did a couple of decades ago, but I don’t think it tells us very much that we didn’t already know about love,” Schwartz said. “It’s kind of interesting, it’s kind of fun [to study]. But do we think that makes us better at love, or helping people with love? Probably not much.”

Love and companionship have made indelible marks on Schwartz and Olds. Though they have separate careers, they’re separate together, working from discrete offices across the hall from each other in their stately Cambridge home. Each has a professional practice and independently trains psychiatry students, but they’ve also collaborated on two books about loneliness and one on marriage. Their own union has lasted 39 years, and they raised two children.

“I think we know a lot more scientifically about love and the brain than we did a couple of decades ago … But do we think that makes us better at love, or helping people with love? Probably not much.” Richard Schwartz, associate professor of psychiatry, Harvard Medical School

“I have learned much more from doing couples therapy, and being in a couple’s relationship” than from science, Olds said. “But every now and again, something like the fMRI or chemical studies can help you make the point better. If you say to somebody, ‘I think you’re doing this, and it’s terrible for a relationship,’ they may not pay attention. If you say, ‘It’s corrosive, and it’s causing your cortisol to go way up,’ then they really sit up and listen.”

A side benefit is that examining other couples’ trials and tribulations has helped their own relationship over the inevitable rocky bumps, Olds said.

“To some extent, being a psychiatrist allows you a privileged window into other people’s triumphs and mistakes,” Olds said. “And because you get to learn from them as they learn from you, when you work with somebody 10 years older than you, you learn what mistakes 10 years down the line might be.”

People have written for centuries about love shifting from passionate to companionate, something Schwartz called “both a good and a sad thing.” Different couples experience that shift differently. While the passion fades for some, others keep its flames burning, while still others are able to rekindle the fires.

“You have a tidal-like motion of closeness and drifting apart, closeness and drifting apart,” Olds said. “And you have to have one person have a ‘distance alarm’ to notice the drifting apart so there can be a reconnection … One could say that in the couples who are most successful at keeping their relationship alive over the years, there’s an element of companionate love and an element of passionate love. And those each get reawakened in that drifting back and forth, the ebb and flow of lasting relationships.”

Children as the biggest stressor

Children remain the biggest stressor on relationships, Olds said, adding that it seems a particular problem these days. Young parents feel pressure to raise kids perfectly, even at the risk of their own relationships. Kids are a constant presence for parents. The days when child care consisted of the instruction “Go play outside” while mom and dad reconnected over cocktails are largely gone.

When not hovering over children, America’s workaholic culture, coupled with technology’s 24/7 intrusiveness, can make it hard for partners to pay attention to each other in the evenings and even on weekends. It is a problem that Olds sees even in environments that ought to know better, such as psychiatry residency programs.

“There are all these sweet young doctors who are trying to have families while they’re in residency,” Olds said. “And the residencies work them so hard there’s barely time for their relationship or having children or taking care of children. So, we’re always trying to balance the fact that, in psychiatry, we stand for psychological good health, but [in] the residency we run, sometimes we don’t practice everything we preach.”

“There is too much pressure … on what a romantic partner should be. They should be your best friend, they should be your lover, they should be your closest relative, they should be your work partner, they should be the co-parent, your athletic partner. … Of course everybody isn’t able to quite live up to it.” Jacqueline Olds, associate professor of psychiatry, Harvard Medical School

All this busy-ness has affected non-romantic relationships too, which has a ripple effect on the romantic ones, Olds said. A respected national social survey has shown that in recent years people have gone from having three close friends to two, with one of those their romantic partner.

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“Often when you scratch the surface … the second [friend] lives 3,000 miles away, and you can’t talk to them on the phone because they’re on a different time schedule,” Olds said. “There is too much pressure, from my point of view, on what a romantic partner should be. They should be your best friend, they should be your lover, they should be your closest relative, they should be your work partner, they should be the co-parent, your athletic partner. There’s just so much pressure on the role of spouse that of course everybody isn’t able to quite live up to it.”

Since the rising challenges of modern life aren’t going to change soon, Schwartz and Olds said couples should try to adopt ways to fortify their relationships for life’s long haul. For instance, couples benefit from shared goals and activities, which will help pull them along a shared life path, Schwartz said.

“You’re not going to get to 40 years by gazing into each other’s eyes,” Schwartz said. “I think the fact that we’ve worked on things together has woven us together more, in good ways.”

Maintain curiosity about your partner

Also important is retaining a genuine sense of curiosity about your partner, fostered both by time apart to have separate experiences, and by time together, just as a couple, to share those experiences. Schwartz cited a study by Robert Waldinger, clinical professor of psychiatry at MGH and HMS, in which couples watched videos of themselves arguing. Afterwards, each person was asked what the partner was thinking. The longer they had been together, the worse they actually were at guessing, in part because they thought they already knew.

“What keeps love alive is being able to recognize that you don’t really know your partner perfectly and still being curious and still be exploring,” Schwartz said. “Which means, in addition to being sure you have enough time and involvement with each other — that that time isn’t stolen — making sure you have enough separateness that you can be an object of curiosity for the other person.”

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The Research on Love: A Psychological, Scientific Perspective on Love

Dr. Nina Mikirova

And now here is this topic about love, the subject without which no movie, novel, poem or song can exist. This topic has fascinated scientists, philosophers, historians, poets, playwrights, novelists, and songwriters. I decided to look at this subject from a scientific point of view. It was very interesting to research and to write this article, and I am hoping it will be interesting to you as a reader.

Whereas psychological science was slow to develop active interest in love, the past few decades have seen considerable growth in research on the subject.  The following is a comprehensive review of the central and well-established findings from psychologically-informed research on love and its influence in adult human relationships as presented in the article: “Love. What Is It, Why Does It Matter, and How Does It Operate?”  by H. Reis and A. Aron. A brief summary of the ideas from this article is presented below.

A BRIEF HISTORY OF LOVE RESEARCH

Most popular contemporary ideas about love can be traced to the classical Greek philosophers. Prominent in this regard is Plato’s Symposium. It is a systematic and seminal analysis whose major ideas

have probably influenced contemporary work on love more than all subsequent philosophical work combined. However, four major intellectual developments of the 19th and 20th centuries provided key insights that helped shape the agenda for current research and theory of love.

The first of these was led by Charles Darwin, who proposed that reproductive success was the central process underlying the evolution of species. Evolutionary theorizing has led directly to such currently popular concepts as mate preference, sexual mating strategies, and attachment, as well as to the adoption of a comparative approach across species.

A second important figure was Sigmund Freud. He introduced many psychodynamic principles, such as the importance of early childhood experiences, the powerful impact of motives operating outside of awareness, the role of defenses in shaping the behavioral expression of motives, and the role of sexuality as a force in human behavior.

A third historically significant figure was Margaret Mead. Mead expanded awareness with vivid descriptions of cultural variations in the expression of love and sexuality. This led researchers to consider the influence of socialization and to recognize cultural variation in many aspects of love.

The emerging women’s movement during the 1970s also contributed to a cultural climate that made the study of what had been traditionally thought of as ‘‘women’s concerns’’ not only acceptable, but in fact necessary for the science of human behavior. At the same time, a group of social psychologists were beginning their work to show that adult love could be studied experimentally and in the laboratory.

WHAT’S PSYCHOLOGY GOT TO DO WITH LOVE

What is Love?  According to authors, Reis and Aron, love is defined as a desire to enter, maintain, or expand a close, connected, and ongoing relationship with another person. Considerable evidence supports a basic distinction, first offered in 1978, between passionate love (“a state of intense longing for union with another”) and other types of romantic love, labeled companionate love (“the affection we feel for those with whom our lives are deeply entwined”).

The evidence for this distinction comes from a variety of research methods, including psychometric techniques, examinations of the behavioral and relationship consequences of different forms of romantic love, and biological studies, which are discussed in this article.  Most work has focused on identifying and measuring passionate love and several aspects of romantic love, which include two components: intimacy and commitment.  Some scholars see companionate love as a combination of intimacy and commitment, whereas others see intimacy as the central component, with commitment as a peripheral factor (but important in its own right, such as for predicting relationship longevity).

In some studies, trust and caring were considred highly prototypical of love, whereas uncertainty and butterflies in the stomach were more peripheral.

Passionate and companionate love solves different adaptation problems. Passionate love may be said to solve the attraction problem—that is, for individuals to enter into a potentially long-term mating relationship, they must identify and select suitable candidates, attract the other’s interest, engage in relationship-building behavior, and then go about reorganizing existing activities and relationships so as to include the other. All of this is strenuous, time-consuming, and disruptive. Consequently, passionate love is associated with many changes in cognition, emotion, and behavior. For the most part, these changes are consistent with the idea of disrupting existing activities, routines, and social networks to orient the individual’s attention and goal-directed behavior toward a specific new partner.

Considerably less study has been devoted to understanding the evolutionary significance of the intimacy and commitment aspects of love. However, much evidence indicates that love in long-term relationships is associated with intimacy, trust, caring, and attachment; all factors that contribute to the maintenance of relationships over time.  More generally, the term companionate love may be characterized by communal relationship; a relationship built on mutual expectations that oneself and a partner will be responsive to each other’s needs.

It was speculated that companionate love, or at least the various processes associated with it, is responsible for the noted association among social relatedness, health, and well-being. In a recent series of papers, it was claimed that marriage is linked to health benefits. Having noted the positive functions of love, it is also important to consider the dark side. That is, problems in love and love relationships are a significant source of suicides, homicides, and both major and minor emotional disorders, such as anxiety and depression. Love matters not only because it can make our lives better, but also because it is a major source of misery and pain that can make life worse.

It is also believed that research will address how culture shapes the experience and expression of love. Although both passionate and companionate love appear to be universal, it is apparent that their manifestations may be moderated by culture-specific norms and rules.

Passionate love and companionate love has profoundly different implications for marriage around the world, considered essential in some cultures but contraindicated or rendered largely irrelevant in others. For example, among U.S. college students in the 1960s, only 24% of women and 65% of men considered love to be the basis of marriage, but in the 1980s this view was endorsed by more than 80% of both women and men.

Finally, the authors believe that the future will see a better understanding of what may be the quintessential question about love: How this very individualistic feeling is shaped by experiences in interaction with particular others.

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Proximate and Ultimate Perspectives on Romantic Love

Geoff kushnick.

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Edited by: Ian D. Stephen, Macquarie University, Australia

Reviewed by: Severi Luoto, The University of Auckland, New Zealand; Cari Goetz, California State University, San Bernardino, United States; Norman Li, Singapore Management University, Singapore

*Correspondence: Adam Bode, [email protected]

† ORCID: Adam Bode, orcid.org/0000-0003-4593-5179 ; Geoff Kushnick, orcid.org/0000-0001-9280-0213

This article was submitted to Evolutionary Psychology, a section of the journal Frontiers in Psychology

Received 2020 Jun 16; Accepted 2021 Mar 12; Collection date 2021.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Romantic love is a phenomenon of immense interest to the general public as well as to scholars in several disciplines. It is known to be present in almost all human societies and has been studied from a number of perspectives. In this integrative review, we bring together what is known about romantic love using Tinbergen’s “four questions” framework originating from evolutionary biology. Under the first question, related to mechanisms, we show that it is caused by social, psychological mate choice, genetic, neural, and endocrine mechanisms. The mechanisms regulating psychopathology, cognitive biases, and animal models provide further insights into the mechanisms that regulate romantic love. Under the second question, related to development, we show that romantic love exists across the human lifespan in both sexes. We summarize what is known about its development and the internal and external factors that influence it. We consider cross-cultural perspectives and raise the issue of evolutionary mismatch. Under the third question, related to function, we discuss the fitness-relevant benefits and costs of romantic love with reference to mate choice, courtship, sex, and pair-bonding. We outline three possible selective pressures and contend that romantic love is a suite of adaptions and by-products. Under the fourth question, related to phylogeny, we summarize theories of romantic love’s evolutionary history and show that romantic love probably evolved in concert with pair-bonds in our recent ancestors. We describe the mammalian antecedents to romantic love and the contribution of genes and culture to the expression of modern romantic love. We advance four potential scenarios for the evolution of romantic love. We conclude by summarizing what Tinbergen’s four questions tell us, highlighting outstanding questions as avenues of potential future research, and suggesting a novel ethologically informed working definition to accommodate the multi-faceted understanding of romantic love advanced in this review.

Keywords: romantic love, mechanisms, ontogeny, functions, phylogeny, Tinbergen, human mating, definition

Introduction

Romantic love is a complex suite of adaptations and by-products that serves a range of functions related to reproduction ( Fletcher et al., 2015 ; Buss, 2019 ). It often occurs early in a romantic relationship but can lead to long-term mating. It is a universal or near-universal ( Jankowiak and Fischer, 1992 ; Gottschall and Nordlund, 2006 ; Jankowiak and Paladino, 2008 ; Fletcher et al., 2015 ; Buss, 2019 ; Sorokowski et al., 2020 ) and is characterized by a range of cognitive, emotional, behavioral, social, genetic, neural, and endocrine activity. It occurs across the lifespan in both sexes. Romantic love serves a variety of functions that vary according to life-stage and duration, including mate choice, courtship, sex, and pair-bonding. Its evolutionary history is probably coupled with the emergence of pair-bonds relatively recently in human evolutionary history.

Romantic love has received attention from scholars in diverse fields, including neurobiology, endocrinology, psychology, and anthropology. Our review aims to synthesize multiple threads of knowledge into a more well-rounded perspective on romantic love. To accomplish this, we do the following: First, we lay out our analytical framework based on Tinbergen’s (1963) “four questions” for explaining a biological phenomenon. Second, using this framework as an organizing tool, we summarize what is known about the social mechanisms, psychological mate choice mechanisms, genetics, neurobiology, endocrinology, development across the lifetime of an individual, fitness-relevant functions, and evolutionary history of romantic love. Finally, we conclude by summarizing what Tinbergen’s four questions tell us, identifying areas for future research, and providing a new ethologically informed working definition of romantic love.

Analytical Framework

Much work has been done to examine romantic love as a biological characteristic. Numerous reviews have described the neurobiology and endocrinology of romantic love (e.g., Fisher, 2004 , 2006 ; Zeki, 2007 ; Hatfield and Rapson, 2009 ; Reynaud et al., 2010 ; Cacioppo et al., 2012b ; de Boer et al., 2012 ; Diamond and Dickenson, 2012 ; Dunbar, 2012 ; Tarlaci, 2012 ; Xu et al., 2015 ; Fisher et al., 2016 ; Zou et al., 2016 ; Tomlinson et al., 2018 ; Walum and Young, 2018 ; Cacioppo, 2019 ). Two meta-analyses ( Ortigue et al., 2010 ; Cacioppo et al., 2012a ) considered fMRI studies of romantic love. There have been some accounts of romantic love or love from an evolutionary perspective (e.g., Hendrick and Hendrick, 1991 ; Fisher, 1995 , 2016 ; Fisher et al., 2006 , 2016 ; Kenrick, 2006 ; Lieberman and Hatfield, 2006 ; Schmitt, 2006 ; Fletcher et al., 2015 ; Sorokowski et al., 2017 ; Buss, 2019 ).

No one, however, has addressed the full spectrum of approaches used in biology to provide a comprehensive account of romantic love. We fill this gap by framing our review of romantic love around Tinbergen’s (1963) “four questions” for explaining biological traits. It was developed in the context of trying to provide a holistic, integrative understanding of animal behavior, and is an extension of earlier explanatory frameworks, including Mayr’s (1961) distinction between proximate and ultimate explanations in biology ( Bateson and Laland, 2013 ). It includes two proximate explanations, mechanistic and ontogenetic, and two ultimate (evolutionary) explanations, functional and phylogenetic. To illustrate the use of this framework, we refer to elements of Zeifman’s (2001) analysis of infant crying as a biological trait using this framework. An outline of our use of this framework is presented in Table 1 .

Summary of romantic love using Tinbergen’s (1963) framework.

Proximate explanations focus on the workings of biological and social systems and their components, both on a short-term (mechanistic) and longer-term (ontogenetic) basis ( Tinbergen, 1963 ; Zeifman, 2001 ). Mechanistic explanations attempt to answer questions about how behavior is produced by an organism. It is about the immediate causation of the behavior. A baby’s cry, under this class of explanation, might be viewed as an expression of emotion regulated by the limbic system. In our analysis, we ask: “What are the mechanisms that cause romantic love?” Ontogenetic explanations attempt to answer questions about how the behavior develops over the life course. A baby’s cry, thus, might be viewed as a vocalization that changes in frequency and context over the first year of life, and then across the rest of childhood. In our analysis, we ask: “How does romantic love develop over the lifetime of an individual?”

Ultimate explanations focus on the application of evolutionary logic to understand behavior, both on a short-term (functional) and long-term (phylogenetic) basis ( Tinbergen, 1963 ; Zeifman, 2001 ). Functional explanations attempt to answer questions about the fitness consequences of behavior and how it functions as an adaptation. A baby’s cry, thus, might be viewed as an adaptation that enhances offspring survival by eliciting care or providing information about its state. As the fitness consequences may be negative as well, it might focus on both benefits and costs. For instance, the cry may decrease survival by attracting predators or depleting scarce energy reserves. In our analysis, we ask: “What are the fitness-relevant functions of romantic love?” Phylogenetic explanations attempt to answer questions about the evolutionary history of a behavior and the mechanisms that produce it. A baby’s cry, thus, might be understood from the perspective of whether similar behaviors are present in closely related species. In our analysis, we ask: “What is the evolutionary history of romantic love?”

Tinbergen’s (1963) framework has been a useful tool for organizing research and theory on behavior and other biological traits across all major kingdoms of life, from plants (e.g., Satake, 2018 ) to humans (e.g., Winterhalder and Smith, 1992 ; Zeifman, 2001 ; Stephen et al., 2017 ; Luoto et al., 2019 ). It allows us to build holistic explanations of biological phenomena by examining complementary, but often non-mutually exclusive, categories of explanation ( Bateson and Laland, 2013 ). We believe that this approach to understanding romantic love will clarify the usefulness and interdependence of the various aspects of the biology of romantic love without falling into the pitfalls of posing explanations for the phenomena that are in opposition rather than complementary ( Nesse, 2013 ).

Definitions

There are a number of definitions and descriptions of romantic love. These definitions and descriptions have different names for romantic love, but all are attempting to define the same construct. We present, here, four definitions or descriptions of romantic love that continue to have relevance to contemporary research.

Walster and Walster (1978) were among the first to scientifically define romantic love. They gave it the name “passionate love” and their definition has been revised several times (e.g., Hatfield and Walster, 1985 ; Hatfield and Rapson, 1993 ). A definition of passionate love is:

A state of intense longing for union with another. Passionate love is a complex functional whole including appraisals or appreciations, subjective feelings, expressions, patterned physiological processes, action tendencies, and instrumental behaviors. Reciprocated love (union with the other) is associated with fulfillment and ecstasy; unrequited love (separation) with emptiness, anxiety, or despair ( Hatfield and Rapson, 1993 , p. 5).

Hendrick and Hendrick (1986) propose a description of romantic love in the context of describing six different “love styles” ( Lee, 1976 ). They label it “eros.” It too has undergone some changes. A recent version of the description is:

Strong physical attraction, emotional intensity, a preferred physical appearance, and a sense of inevitability of the relationship define the central core of eros. Eros can “strike” suddenly in a revolution of feeling and thinking ( Hendrick and Hendrick, 2019 , p. 244).

Sternberg (1986) provides a description of romantic love based on three components of love in close relationships: intimacy, passion and commitment. He calls it “romantic love” and describes it as such:

This kind of love derives from a combination of the intimacy and passion components of love. In essence, it is liking with an added element, namely, the arousal brought about by physical attraction and its concomitants. According to this view, then, romantic lovers are not only drawn physically to each other but are also bonded emotionally ( Sternberg, 1986 , p. 124).

A more recent definition of romantic love informed by evolutionary theory has been proposed by Fletcher et al. (2015) . Rather than providing a discrete series of sentences, they propose a working definition of “romantic love” that is explained with reference to some of the psychological research on romantic love and by summarizing five distinct features of romantic love. These features are:

Romantic love is a powerful commitment device, composed of passion, intimacy, and caregiving;

Romantic love is universal and is associated with pair-bonding across cultures;

Romantic love automatically suppresses effort and attention given to alternative partners;

Romantic love has distinct emotional, behavioral, hormonal, and neuropsychological features; and

Successful pair-bonding predicts better health and survival across cultures for both adults and offspring ( Fletcher et al., 2015 , p. 22).

Despite these attempts to define and describe romantic love, no single term or definition has been universally adopted in the literature. The psychological literature often uses the terms “romantic love,” “love,” and “passionate love” (e.g., Sternberg and Sternberg, 2019 ). Seminal work called it “limerence” ( Tennov, 1979 ). The biological literature generally uses the term “romantic love” and has investigated “early stage intense romantic love” (e.g., Xu et al., 2011 ), “long-term intense romantic love” (e.g., Acevedo et al., 2012 ), or being “in love” (e.g., Marazziti and Canale, 2004 ). In this review, what we term “romantic love” encompasses all of these definitions, descriptions, and terms. Romantic love contrasts with “companionate love,” which is felt less intensely, often follows a period of romantic love ( Hatfield and Walster, 1985 ), and merges feelings of intimacy and commitment ( Sternberg, 1986 ).

Psychological Characteristics

Hatfield and Sprecher (1986) theoretically developed the Passionate Love Scale to assess the cognitive, emotional, and behavioral components of romantic love among people who are in a relationship. There are other ways of measuring romantic love ( Hatfield et al., 2012 ), and some, such as Sternberg’s Triangular Love Scale ( Sternberg, 1997 ; Sumter et al., 2013 ) or the Love Attitudes Scale ( Hendrick and Hendrick, 1986 ; Hendrick et al., 1998 ), measure the same constructs ( Masuda, 2003 ; Graham, 2011 ). The Passionate Love Scale is only valid in people who are in a romantic relationship with their loved one. Regardless, the Passionate Love Scale provides a particularly useful account of some of the psychological characteristics of romantic love. It has been used widely in research investigating romantic love in relationships ( Feybesse and Hatfield, 2019 ).

Cognitive components of romantic love include intrusive thinking or preoccupation with the partner, idealization of the other in the relationship, and desire to know the other and to be known. Emotional components include attraction to the other, especially sexual attraction, negative feelings when things go awry, longing for reciprocity, desire for complete union, and physiological arousal. Behavioral components include actions toward determining the other’s feelings, studying the other person, service to the other, and maintaining physical closeness ( Hatfield and Sprecher, 1986 ).

Romantic love shares a number of physiological and psychological characteristics with addiction. “[T]hey focus on their beloved (salience); and they yearn for their beloved (craving). They feel a “rush” of exhilaration when seeing or thinking about him or her (euphoria/intoxication). As their relationship builds, the lover experiences the common signs of drug withdrawal, too, including protest, crying spells, lethargy, anxiety, insomnia, or hypersomnia, loss of appetite or binge eating, irritability and chronic loneliness.” ( Fisher et al., 2016 , p. 2) A number of reviews have highlighted the behavioral and neurobiological similarities between addiction and romantic love (e.g., Reynaud et al., 2010 ; Fisher et al., 2016 ; Zou et al., 2016 ).

There is evidence that romantic love is associated with increased hypomanic symptoms (elevated mood, Brand et al., 2007 ; Bajoghli et al., 2011 , 2013 , 2014 , 2017 ; Brand et al., 2015 ), a change (increase or decrease) in depression symptoms ( Stoessel et al., 2011 ; Bajoghli et al., 2013 , 2014 , 2017 ; Price et al., 2016 ; Verhallen et al., 2019 ; Kuula et al., 2020 ), and increased state anxiety ( Hatfield et al., 1989 ; Wang and Nguyen, 1995 ; Bajoghli et al., 2013 , 2014 , 2017 ; Brand et al., 2015 ; Kuula et al., 2020 ). See Supplementary Table 1 for information about studies investigating hypomania, depression, and anxiety symptoms in people experiencing romantic love. Romantic love is also characterized by cognitive biases which resemble “positive illusions,” which are a tendency to perceive one’s relationship and one’s loved one in a positive light or bias ( Song et al., 2019 ).

Proximate Perspectives

When applied to romantic love, the first of Tinbergen’s (1963) four questions asks: “What are the mechanisms that cause romantic love?” This can be answered with reference to social mechanisms, psychological mate choice mechanisms, genetics, neurobiology, and endocrinology ( Zeifman, 2001 ; Bateson and Laland, 2013 ). Research into the social mechanisms and genetics of romantic love are in their infancy, but there is substantial theory on psychological mate choice mechanisms and ample research has been undertaken into the neural and endocrine activity associated with romantic love. Additional insights can be garnered from the neurobiology and endocrinology of psychopathology, cognitive biases, and animal models.

Social Mechanisms

Some precursors to romantic love (others discussed below) that act strongly as social mechanisms that cause romantic love are reciprocal liking, propinquity, social influence, and the filling of needs (e.g., Aron et al., 1989 ; Pines, 2001 ; Riela et al., 2010 ). Reciprocal liking (mutual attraction) is “being liked by the other, both in general, as well as when it is expressed through self-disclosure” ( Aron et al., 1989 , p. 245). It has been frequently identified as preceding romantic love among participants from the United States and is cross-culturally identified as the strongest preference in mates among both sexes ( Buss et al., 1990 ). “Whether expressed in a warm smile or a prolonged gaze, the message is unmistakable: ‘It’s safe to approach, I like you too. I’ll be nice. You’re not in danger of being rejected”’ ( Hazan and Diamond, 2000 , p. 197). Reciprocal liking may encourage the social approach and courtship activities characteristic and causative of romantic love.

Propinquity is “familiarity, in terms of having spent time together, living near the other, mere exposure to the other, thinking about the other, or anticipating interaction with the other” ( Aron et al., 1989 , p. 245). It has more recently been named “familiarity” (see Riela et al., 2010 ). The extended exposure of an individual to another helps to cause romantic love and specifically facilities the development of romantic love over extended periods of time. Propinquity, in our evolutionary history, served to ensure that “potential mates who are encountered daily at the river’s edge have an advantage over those residing on the other side” ( Hazan and Diamond, 2000 , p. 201). Given that the pool of potential mates in our evolutionary history would have been limited by the size of the groups in which we lived and the fact that most individuals of reproductive age would already have been involved in long-term mating relationships, propinquity is likely to have played a particularly important role in the generation of romantic love. Until recently (to a somewhat lesser extent, today), with the wide-scale take-up of online dating, propinquity played a role in the formation of many long-term pair-bonds, and presumably, romantic love, as is evidenced by a relatively high proportion of people having met their romantic partners in the places where exposure was facilitated, such as school, college, or work ( Rosenfeld et al., 2019 ). Changes in the importance of certain precursors in causing romantic love may be the result of a mismatch between the modern environment and our genotypes that evolved in a very different environment (discussed in detail below; see Li et al., 2018 ).

Social influences are “both general social norms and approval of others in the social network” ( Aron et al., 1989 , p. 245). This may cause people to fall in love with others who are of a similar attractiveness, cultural group, ethnic group, profession, economic class, or who are members of the same social group. Social influences may, directly, impact who we fall in love with by providing approval to a romantic union or, indirectly, by facilitating propinquity. The effect of social influences is demonstrated in the relatively large number of people who met their romantic partner through friends ( Rosenfeld et al., 2019 ). The filling of needs is “having the self’s needs met or meeting the needs of the other (e.g., he makes me happy, she buys me little presents that show she cares), and typically implies characteristics that are highly valued and beneficial in relationship maintenance (e.g., compassion, respect)” ( Riela et al., 2010 , pp. 474–475). The filling of needs may cause romantic love when social interaction facilitates a union where both partners complement each other.

Psychological Mate Choice Mechanisms

Mate choice, in the fields of evolutionary theory, can be defined as “the process that occurs whenever the effects of traits expressed in one sex lead to non-random allocation of reproductive investment with members of the opposite sex” ( Edward, 2015 , p. 301). It is essentially the process of intersexual selection proposed by Darwin (2013) more than 150 years ago ( Darwin, 1859 ) whereby someone has a preference for mating with a particular individual because of that individual’s characteristics. Mate choice, to that extent, involves the identification of a desirable conspecific ( Fisher et al., 2005 ) and sometimes, the focusing of mating energies on that individual. Mate preferences, sexual desire, and attraction all contribute to romantic love. The concepts of “extended phenotypes” and “overall attractiveness” help to explain how these features operate. Romantic love, as discussed below, serves a mate choice function ( Fisher et al., 2005 ) and these mechanisms and constructs contribute to when, and with whom, an individual falls in love.

A large body of research has developed around universal mate preferences (e.g., Buss and Barnes, 1986 ; Buss, 1989 ; Buss et al., 1990 ; Buss and Schmitt, 2019 ; Walter et al., 2020 ). Women, more than men, show a strong preference for resource potential, social status, a slightly older age, ambition and industriousness, dependability and stability, intelligence, compatibility, certain physical indicators, signs of good health, symmetry, masculinity, love, kindness, and commitment ( Buss, 1989 , 2016 ; Walter et al., 2020 ). Men, more than women, have preferences for youth, physical beauty, certain body shapes, chastity, and fidelity ( Buss, 1989 , 2016 ). Both sexes have particularly strong preferences for kindness and intelligence ( Buss et al., 1990 ). A male-taller-than-female norm exists in mate preferences and there is some evidence that women have a preference for taller-than-average height (e.g., Salska et al., 2008 ; Yancey and Emerson, 2014 ). Mutual attraction and reciprocated love are the most important characteristics that both women and men look for in a potential partner ( Buss et al., 1990 ).

Mate choice and attraction may be based on assessments of “extended phenotypes” ( Dawkins, 1982 ; Luoto, 2019a ), which include biotic and abiotic features of the environment that are influenced by an individual’s genes. For example, an extended phenotype would include an individual’s dwelling, car, pets, and social media presence. These can convey information relevant to fitness. Overall mate attractiveness, which is constituted by signs of health and fertility, neurophysiological efficiency, provisioning ability and resources, and capacity for cooperative relationships ( Miller and Todd, 1998 ) may be another heuristic through which attraction and mate choice operate.

Many mate preferences are relatively universal and therefore are likely to have at least some genetic basis (as suggested by, Sugiyama, 2015 ). While mate preferences are linked to actual mate selection ( Li et al., 2013 ; Li and Meltzer, 2015 ; Conroy-Beam and Buss, 2016 ; Buss and Schmitt, 2019 ), strong mate preferences do not always translate into real-world mate choice ( Todd et al., 2007 ; Stulp et al., 2013 ). This is in part because mate preferences function in a tradeoff manner whereby some preferences are given priority over others (see Li et al., 2002 ; Thomas et al., 2020 ). That is, mate choice is a multivariate process that includes the integration and tradeoff of several preferences ( Conroy-Beam et al., 2016 ). Mate preferences are important because they may serve as a means of screening potential mates, while sexual desire and attraction operationalize these preferences, and romantic love crystalizes them.

Sexual desire and attraction may be antecedents to falling in love and there is evidence that physiologically, sexual desire progresses into romantic love within shared neural structures ( Cacioppo et al., 2012a ). However, although both sexual desire and attraction operationalize mate choice, only attraction, and not sexual desire, may be necessary for romantic love to occur (see Leckman and Mayes, 1999 ; Diamond, 2004 ). Intense attraction is characterized by increased energy, focused attention, feelings of exhilaration, intrusive thinking, and a craving for emotional union ( Fisher, 1998 ) although it exists on a spectrum of intensity.

Changes in the expression of at least 61 genes are associated with falling in love in women ( Murray et al., 2019 ) suggesting that these genes may regulate features of romantic love. The DRD2 Taq I A polymorphism, which regulates Dopamine 2 receptor density ( Jonsson et al., 1999 ), is associated with eros ( Emanuele et al., 2007 ). Polymorphisms of genes that regulate vasopressin receptors (AVPR1a rs3), oxytocin receptors (OXTR rs53576), dopamine 4 receptors (DRD4-7R), and dopamine transmission (COMT rs4680) are associated with activity in the ventral tegmental area which, in turn, is associated with eros in newlyweds ( Acevedo et al., 2020 ).

Neurobiology

Neuroimaging studies (see Supplementary Table 2 ) implicate dozens of brain regions in romantic love. We focus, here, on only some of the most frequently replicated findings in an attempt to simplify a description of the neural activity associated with romantic love and explain its psychological characteristics. Romantic love, at least in people who are in a relationship with their loved one, appears to be associated with activity (activation or deactivation compared with a control condition) in four main overlapping systems: reward and motivation, emotions, sexual desire and arousal, and social cognition.

Reward and motivation structures associated with romantic love include those found in the mesolimbic pathway: the ventral tegmental area, nucleus accumbens, amygdala, and medial prefrontal cortex ( Xu et al., 2015 ). Activity in the mesolimbic pathway substantiates the claim that romantic love is a motivational state ( Fisher et al., 2005 ) and helps to explain why romantic love is characterized by psychological features such as longing for reciprocity, desire for complete union, service to the other, maintaining physical closeness, and physiological arousal ( Hatfield and Sprecher, 1986 ).

Emotional centers of the brain associated with romantic love include the amygdala, the anterior cingulate cortex ( Bartels and Zeki, 2000 ; Aron et al., 2005 ; Fisher et al., 2010 ; Younger et al., 2010 ; Zeki and Romaya, 2010 ; Stoessel et al., 2011 ; Acevedo et al., 2012 ; Scheele et al., 2013 ; Song et al., 2015 ), and the insula ( Bartels and Zeki, 2000 ; Aron et al., 2005 ; Ortigue et al., 2007 ; Fisher et al., 2010 ; Younger et al., 2010 ; Zeki and Romaya, 2010 ; Stoessel et al., 2011 ; Acevedo et al., 2012 ; Xu et al., 2012b ; Song et al., 2015 ). Activity in these structures helps to explain romantic love’s emotional features such as negative feelings when things go awry, longing for reciprocity, desire for complete union, and physiological arousal ( Hatfield and Sprecher, 1986 ).

The primary areas associated with both romantic love and sexual desire and arousal include the caudate, insula, putamen, and anterior cingulate cortex ( Diamond and Dickenson, 2012 ). The involvement of these regions helps to explain why people experiencing romantic love feel extremely sexually attracted to their loved one ( Hatfield and Sprecher, 1986 ). The neural similarities and overlapping psychological characteristics of romantic love and sexual desire are well documented (see Hatfield and Rapson, 2009 ; Cacioppo et al., 2012a ; Diamond and Dickenson, 2012 ).

Social cognition centers in the brain repeatedly associated with romantic love include the amygdala, the insula ( Adolphs, 2001 ), and the medial prefrontal cortex ( Van Overwalle, 2009 ). Social cognition plays a role in the social appraisals and cooperation characteristics of romantic love. Activity in these regions helps to explain psychological characteristics such as actions toward determining the other’s feelings, studying the other person, and service to the other ( Hatfield and Sprecher, 1986 ).

In addition to activity in these four systems, romantic love is associated with activity in higher-order cortical brain areas that are involved in attention, memory, mental associations, and self-representation ( Cacioppo et al., 2012b ). Mate choice (a function of romantic love detailed below) has been specifically associated with the mesolimbic pathway and hypothalamus ( Calabrò et al., 2019 ). The mesolimbic pathway, thalamus, hypothalamus, amygdala, septal region, prefrontal cortex, cingulate cortex, and insula have been specifically associated with human sexual behavior ( Calabrò et al., 2019 ), which has implications for the sex function of romantic love (detailed below).

Isolated studies have identified sex differences in the neurobiological activity associated with romantic love. One study ( Bartels and Zeki, 2004 ) found activity in the region ventral to the genu in only women experiencing romantic love. One preliminary study of romantic love (see Fisher et al., 2006 ) found that “[m]en tended to show more activity than women in a region of the right posterior dorsal insula that has been correlated with penile turgidity and male viewing of beautiful faces. Men also showed more activity in regions associated with the integration of visual stimuli. Women tended to show more activity than men in regions associated with attention, memory and emotion” (p. 2181).

Endocrinology

Romantic love is associated with changes in circulating sex hormones, serotonin, dopamine, oxytocin, cortisol, and nerve growth factor systems. Table 2 presents the endocrine factors which are found to be different, compared to controls, in people experiencing romantic love. More information about the controlled studies discussed in this subsection is presented in Supplementary Table 3 . Endocrine factors associated with romantic love have most of their psychological and other effects because of their role as a hormone (e.g., sex hormones, cortisol) or neurotransmitter (e.g., serotonin, dopamine), although many factors operate as both (see Calisi and Saldanha, 2015 ) or as neurohormones.

Significant results of controlled endocrine studies investigating romantic love.

<, lower level than control; >, higher level than controls; FSH, follicle-stimulating hormone; NGF, nerve growth factor; LH, luteinizing hormone; *, decreased transporter density is indicative of higher extracellular neurotransmitter levels.

Romantic love is associated with changes in the sex hormones testosterone, follicle-stimulating hormone, and luteinizing hormone ( Marazziti and Canale, 2004 ; Durdiakova et al., 2017 ; Sorokowski et al., 2019 ), although the findings have been inconsistent. Testosterone appears to be lower in men experiencing romantic love than controls ( Marazziti and Canale, 2004 ) and higher eros scores are associated with lower levels of testosterone in men ( Durdiakova et al., 2017 ). Lower levels of testosterone in fathers are associated with greater involvement in parenting (see Storey et al., 2020 , for review). The direction of testosterone change in women is unclear (see Marazziti and Canale, 2004 ; Sorokowski et al., 2019 ). Sex hormones are involved in the establishment and maintenance of sexual characteristics, sexual behavior, and reproductive function ( Mooradian et al., 1987 ; Chappel and Howles, 1991 ; Holloway and Wylie, 2015 ). Some sex hormones can influence behavior through their organizing effects resulting from prenatal and postnatal exposure. In the case of romantic love, however, the effects of sex hormones on the features of romantic love are the result of activating effects associated with behaviorally contemporaneous activity. It is possible that sex hormones influence individual differences in the presentation of romantic love through their organizing effect (see Motta-Mena and Puts, 2017 ; Luoto et al., 2019 ; Arnold, 2020 ; McCarthy, 2020 , for descriptions of organizing and activating effects of testosterone, estradiol, and progesterone). Changes in sex hormones could help to explain the increase in sexual desire and arousal associated with romantic love ( Hatfield and Sprecher, 1986 ; Hatfield and Rapson, 2009 ; Diamond and Dickenson, 2012 ).

Romantic love is associated with decreased serotonin transporter density ( Marazziti et al., 1999 ) and changes in plasma serotonin ( Langeslag et al., 2012 ), although inconsistencies have been found in the direction of change according to sex. In one study, men experiencing romantic love displayed lower serotonin levels than controls and women displayed higher serotonin levels than controls ( Langeslag et al., 2012 ). Decreased serotonin transporter density is indicative of elevated extracellular serotonin levels ( Mercado and Kilic, 2010 ; Jørgensen et al., 2014 ). However, decreased levels of serotonin are thought to play a role in depression, mania, and anxiety disorders ( Mohammad-Zadeh et al., 2008 ), including obsessive-compulsive disorder (for a discussion of the relationship between serotonin and OCD, see Baumgarten and Grozdanovic, 1998 ; Rantala et al., 2019 ). One study showed that a sample of mainly women (85% women) experiencing romantic love have similar levels of serotonin transporter density to a sample of both women and men (50% women) with obsessive-compulsive disorder ( Marazziti et al., 1999 ), which could account for the intrusive thinking or preoccupation with the loved one associated with romantic love ( Hatfield and Sprecher, 1986 ).

Lower dopamine transporter density and lower dopamine transporter maximal velocity in lymphocytes have been found in people experiencing romantic love ( Marazziti et al., 2017 ). This is indicative of increased dopamine levels ( Marazziti et al., 2017 ) and is consistent with neuroimaging studies (e.g., Takahashi et al., 2015 ; Acevedo et al., 2020 ) showing activation of dopamine-rich regions of the mesolimbic pathway. One study ( Dundon and Rellini, 2012 ) found no difference in dopamine levels in urine in women experiencing romantic love compared with a control group. Dopamine is involved in reward behavior, sleep, mood, attention, learning, pain processing, movement, emotion, and cognition ( Ayano, 2016 ). Up-regulation of the dopamine system could help to explain the motivational characteristics of romantic love such as longing for reciprocity, desire for complete union, service to the other, and maintaining physical closeness ( Hatfield and Sprecher, 1986 ).

There are no studies that have specifically investigated oxytocin levels in romantic love (at least none that measure romantic love with a validated scale). However, studies ( Schneiderman et al., 2012 , Schneiderman et al., 2014 ; Ulmer-Yaniv et al., 2016 ) have demonstrated that people in the early stages of their romantic relationship have higher levels of plasma oxytocin than controls (singles and new parents). We infer this to mean that reciprocated romantic love is associated with elevated oxytocin levels. Oxytocin plays a role in social affiliation ( IsHak et al., 2011 ) and pair-bonding ( Young et al., 2011 ; Acevedo et al., 2020 ). Oxytocin receptors are prevalent throughout the brain including in the mesolimbic pathway (e.g., Bartels and Zeki, 2000 ). Elevated oxytocin could account for many of the behavioral features of romantic love such as actions toward determining the other’s feelings, studying the other person, service to the other, and maintaining physical closeness ( Hatfield and Sprecher, 1986 ).

Romantic love has been associated with elevated cortisol levels ( Marazziti and Canale, 2004 ), although this has not been replicated ( Sorokowski et al., 2019 ), and one study measuring cortisol in saliva found the opposite ( Weisman et al., 2015 ). Different results could be attributed to different length of time in a relationship between the samples (see Garcia, 1997 ; de Boer et al., 2012 ). Cortisol plays a role in the human stress response by directing glucose and other resources to various areas of the body involved in responding to environmental stressors while simultaneously deactivating other processes (such as digestion and immune regulation, Mercado and Hibel, 2017 ). Elevated cortisol levels may play a role in pair-bond initiation ( Mercado and Hibel, 2017 ) and are indicative of a stressful environment.

Romantic love is associated with higher levels of nerve growth factor, and the intensity of romantic love correlates with levels of nerve growth factor ( Emanuele et al., 2006 ). Nerve growth factor is a neurotrophic implicated in psycho-neuroendocrine plasticity and neurogenesis ( Berry et al., 2012 ; Aloe et al., 2015 ; Shohayeb et al., 2018 ) and could contribute to some of the neural and endocrine changes associated with romantic love.

Insights From the Mechanisms of Psychopathology

Despite “madness” being mentioned in one review of the neurobiology of love ( Zeki, 2007 ) and psychopathology being discussed in studies investigating the endocrinology of romantic love (e.g., Marazziti et al., 1999 , 2017 ), the similarities between romantic love and psychopathology are under-investigated. An understanding of the mechanisms that regulate addiction, mood disorders, and anxiety disorders may help to shed light on the psychological characteristics and mechanisms underlying romantic love and identify areas for future research.

Conceptualizing romantic love as a “natural addiction” (e.g., Fisher et al., 2016 ) not only helps to explain romantic love’s psychological characteristics but provides insight into the mechanisms underlying it (e.g., Zou et al., 2016 ). For example, a neurocircuitry analysis of addiction, drawing on human and animal studies, reveals mechanisms of different “stages” of addiction that have implications for romantic love: binge/intoxication (encompassing drug reward and incentive salience), withdrawal/negative affect, and preoccupation/anticipation ( Koob and Volkow, 2016 ). Each of these stages is associated with particular neurobiological activity and each stage could be represented in romantic love. This may mean that the findings of studies investigating the neurobiology of romantic love (which rely primarily on studies where visual stimuli of a loved one are presented) equates to the binge/intoxication stage of addiction. Findings from studies investigating romantic rejection ( Fisher et al., 2010 ; Stoessel et al., 2011 ; Song et al., 2015 ) may equate to the withdrawal/negative affect stage of addiction. Findings from resting-state fMRI studies ( Song et al., 2015 ; Wang et al., 2020 ) may equate to the preoccupation/anticipation stage of addiction. The result is that current neuroimaging studies may paint a more detailed picture of the neurobiology of romantic love than might initially be assumed.

Mood is an emotional predictor of the short-term prospects of pleasure and pain ( Morris, 2003 ). The adaptive function of mood is, essentially, to integrate information about the environment and state of the individual to fine-tune decisions about behavioral effort ( Nettle and Bateson, 2012 ). Elevated mood can serve to promote goal-oriented behavior and depressed mood can serve to extinguish such behavior ( Wrosch and Miller, 2009 ; Bindl et al., 2012 ; Nesse, 2019 ). Anxious mood is a response to repeated threats ( Nettle and Bateson, 2012 ). Because romantic love can be a tumultuous time characterized by emotional highs, lows, fear, and trepidation, and can involve sustained and repetitive efforts to pursue and retain a mate, it follows that mood circuitry would be closely intertwined with romantic love. Additionally, because romantic love concerns itself with reproduction, which is the highest goal in the realm of evolutionary fitness, it makes sense that mood may impact upon the way romantic love manifests. Understanding the mechanisms that regulate mood can provide insights into psychological characteristics of romantic love and the mechanisms that regulate it. No studies have directly investigated the mechanisms that contribute to changes in mood in people experiencing romantic love. However, insights can be taken from research into the mechanisms of mood and anxiety disorders.

While addiction, hypomania, depression, and anxiety symptoms in people experiencing romantic love may be the normal manifestation of particular mechanisms, symptoms associated with psychopathology may be the manifestations of malfunctioning mechanisms as a result of evolutionary mismatch (see Durisko et al., 2016 ; Li et al., 2018 ). As a result, the mechanisms that cause romantic love and those that cause psychopathology may not be precise models with which to investigate the other. Nonetheless, the mechanisms that cause psychopathology may provide a useful framework with which to base future research into romantic love. Conversely, it may also be that our understanding of the mechanisms that cause romantic love could be a useful framework with which to further investigate psychopathology.

The drug reward and incentive salience features of the binge/intoxication stage of addiction involve changes in dopamine and opioid peptides in the basal ganglia (i.e., striatum, globus pallidus, subthalamic nucleus, and substantia nigra pars reticulata, Koob and Volkow, 2016 ). No research has investigated opioids in romantic love, despite them being involved in monogamy in primates (see French et al., 2018 ) and pair-bonding in rodents ( Loth and Donaldson, 2021 ). The negative emotional states and dysphoric and stress-like responses in the withdrawal/negative affect stage are caused by decreases in the function of dopamine in the mesolimbic pathway and recruitment of brain stress neurotransmitters (i.e., corticotropin-releasing factor, dynorphin), in the extended amygdala ( Koob and Volkow, 2016 ). No studies have investigated corticotropin-releasing factor in romantic love. The craving and deficits in executive function in the preoccupation/anticipation stage of addiction involve the dysregulation of projections from the prefrontal cortex and insula (e.g., glutamate), to the basal ganglia and extended amygdala ( Koob and Volkow, 2016 ). No studies have investigated glutamate in romantic love. There are at least 18 neurochemically defined mini circuits associated with addiction ( Koob and Volkow, 2016 ) that could be the target of research into romantic love. It is likely that romantic love has similar, although not identical, mechanisms to addiction (see Zou et al., 2016 ; Wang et al., 2020 ).

Mania/hypomania (bipolar disorder)

Similar to the brain regions implicated in romantic love, the ventral tegmental area has been associated with mania ( Abler et al., 2008 ), the ventral striatum has been associated with bipolar disorder ( Dutra et al., 2015 ), and the amygdala has been associated with the development of bipolar disorder ( Garrett and Chang, 2008 ). These findings should be interpreted with caution, however, as replicating neuroimaging findings in bipolar disorder has proven difficult (see Maletic and Raison, 2014 ). Research implicates two interrelated prefrontal–limbic networks in elevated mood, which overlap with activity found in romantic love: the automatic/internal emotional regulatory network which includes the ventromedial prefrontal cortex, subgenual anterior cingulate cortex, nucleus accumbens, globus pallidus, and the thalamus, and the volitional/external regulatory network which includes the ventrolateral prefrontal cortex, mid- and dorsal-cingulate cortex, ventromedial striatum, globus pallidus, and thalamus ( Maletic and Raison, 2014 ).

Norepinephrine (theorized to be involved in romantic love, e.g., Fisher, 1998 , 2000 ), serotonin, dopamine, and acetylcholine play a role in bipolar disorder ( Manji et al., 2003 ). One study ( Dundon and Rellini, 2012 ) found no difference in norepinephrine levels in urine in women experiencing romantic love compared with a control group. No studies have investigated acetylcholine in romantic love but romantic love is associated with serotonin ( Marazziti et al., 1999 ; Langeslag et al., 2012 ) and dopamine activity ( Marazziti et al., 2017 ). Similar to the endocrine factors implicated in romantic love ( Emanuele et al., 2006 ; Schneiderman et al., 2012 , 2014 ; Ulmer-Yaniv et al., 2016 ), bipolar patients in a period of mania have also demonstrated higher oxytocin ( Turan et al., 2013 ) and nerve growth factor ( Liu et al., 2014 ) levels and lower levels of serotonin ( Shiah and Yatham, 2000 ). Additionally, there is some evidence that women diagnosed with bipolar disorder present with higher levels of testosterone whereas men present with lower levels of testosterone compared with sex-matched controls ( Wooderson et al., 2015 ). Similar findings have been found in romantic love ( Marazziti and Canale, 2004 ). Dysfunction in the hypothalamic–pituitary–adrenal axis, where cortisol plays a major role, has also been implicated in bipolar disorder ( Maletic and Raison, 2014 ). Cortisol probably plays a role in romantic love ( Marazziti and Canale, 2004 ; Weisman et al., 2015 ).

Neuroimaging studies have implicated changes in functional connectivity in the neural circuits involved in affect regulation in people experiencing depression ( Dean and Keshavan, 2017 ). Increased functional connectivity has been found in networks involving some of the same regions, such as the amygdala, the medial prefrontal cortex, and nucleus accumbens in both people experiencing romantic love and people who recently ended their relationship while in love ( Song et al., 2015 ).

There are a number of endocrine similarities between romantic love and depression. One major pathophysiological theory of depression is that it is caused by an alteration in levels of one or more monoamines, including serotonin, norepinephrine, and dopamine ( Dean and Keshavan, 2017 ). Altered dopamine transmission in depression may be characterized by a down-regulated dopamine system (see Belujon and Grace, 2017 ), which is inferred from numerous human and animal studies, including successful treatment in humans with a dopamine agonist. In romantic love, however, dopamine appears to be up-regulated, especially in areas of the mesolimbic pathway (e.g., Marazziti et al., 2017 ; Bartels and Zeki, 2000 ; Acevedo et al., 2020 ). This could account for some findings that romantic love is associated with a reduction in depression symptoms ( Bajoghli et al., 2013 , 2017 ). However, these need to be reconciled with contrasting findings that romantic love is associated with increased depression symptoms ( Bajoghli et al., 2014 ; Kuula et al., 2020 ) and evidence suggesting that a relationship breakup in people experiencing romantic love is associated with depression symptoms ( Stoessel et al., 2011 ; Price et al., 2016 ; Verhallen et al., 2019 ). The mechanisms that underlie depression might provide a framework for such efforts.

Dysregulation of the HPA axis and associated elevated levels of cortisol is theorized to be one contributor to depression ( Dean and Keshavan, 2017 ). Changes in oxytocin and vasopressin systems (theorized to be involved in romantic love, e.g., Fisher, 1998 , 2000 ; Carter, 2017 ; Walum and Young, 2018 ) are associated with depression (see Purba et al., 1996 ; Van Londen et al., 1998 ; Neumann and Landgraf, 2012 ; McQuaid et al., 2014 ). No studies have investigated vasopressin in people experiencing romantic love. There is also decreased neurogenesis and neuroplasticity in people experiencing depression ( Dean and Keshavan, 2017 ), the opposite of which can be inferred to occur in romantic love because of its substantial neurobiological activity and elevated nerve growth factor (see Berry et al., 2012 ; Aloe et al., 2015 ; Shohayeb et al., 2018 ).

The insular cortex, cingulate cortex, and amygdala are implicated in anxiety and anxiety disorders ( Martin et al., 2009 ). There is also evidence that cortisol, serotonin and norepinephrine are involved ( Martin et al., 2009 ). The substantial overlap between the mechanisms regulating romantic love and those causing anxiety and anxiety disorders provides an opportunity to investigate specific mechanistic effects on the psychological characteristics of romantic love. Assessing state anxiety and these mechanisms concurrently in people experiencing romantic love may be a fruitful area of research.

There is also a need to clarify the role of the serotonin system in romantic love. Similar serotonin transporter density in platelets in people experiencing romantic love and OCD suggests a similar serotonin-related mechanism in both ( Marazziti et al., 1999 ). However, lower serotonin transporter density in platelets is indicative of higher extracellular serotonin levels ( Mercado and Kilic, 2010 ; Jørgensen et al., 2014 ). This is despite lower levels of serotonin being theorized to contribute to anxiety ( Mohammad-Zadeh et al., 2008 ). One study found lower circulating serotonin levels in men experiencing romantic love than controls and higher levels of circulating levels of serotonin in women experiencing romantic love than controls ( Langeslag et al., 2012 ). Insights from the mechanisms regulating anxiety disorders may help to provide a framework with which to further investigate the role of the serotonin system in romantic love and reconcile these findings.

Insights From Cognitive Biases

Positive illusions are cognitive biases about a relationship and loved one that are thought to have positive relationship effects ( Song et al., 2019 ). The research into positive illusions does not use samples of people explicitly experiencing romantic love, and instead uses people in varied stages of a romantic relationship, including those in longer-term pair-bonds. One study ( Swami et al., 2009 ), however, did find a correlation between the “love-is-blind bias” (one type of positive illusion) and eros scores. We also know that cognitive biases resembling positive illusions do exist in romantic love. Both the Passionate Love Scale (e.g., “For me, ____ is the perfect romantic partner,” Hatfield and Sprecher, 1986 , p. 391) and the eros subscale of the Love Attitudes Scale (e.g., “My lover fits my ideal standards of physical beauty/handsomeness,” Hendrick and Hendrick, 1986 , p. 395) include questions about a respondent’s loved one that resemble measures of positive illusions. Understanding the mechanism that regulates positive illusions will provide a model against which the mechanisms regulating the cognitive features of romantic love can be assessed.

A proposed mechanism of positive illusions includes the caudate nucleus, dorsal anterior cingulate cortex, ventral anterior cingulate cortex, orbitofrontal cortex, ventrolateral prefrontal cortical regions, and dorsal medial prefrontal cortex ( Song et al., 2019 ). These regions overlap with the brain regions associated with romantic love. This suggests that the cognitive biases associated with romantic love may be related to, but are distinct from, positive illusions. Targeted neuroimaging studies could ascertain any involvement of the ventrolateral prefrontal cortex and the dorsal medial prefrontal cortex in romantic love. Such research could help to delineate a mechanism that specifically regulates one cognitive aspect of romantic love from those that regulate other psychological aspects of romantic love.

Insights From Mammalian Pair-Bonding Mechanisms

It is not possible to say with any certainty if other animals experience romantic love. Some certainly engage in pair-bonding and exhibit behaviors that are characteristic of romantic love such as obsessive following, affiliative gestures, and mate guarding (see Fisher et al., 2006 ). While some similarities between humans and other animals may be the result of parallel evolution, an understanding of the mechanisms involved in pair-bond formation in other animals can raise questions and guide research into romantic love in humans. Research into monogamous prairie voles, in particular, has identified neurobiological and endocrinological mechanisms that regulate pair-bonding processes. Drawing on this research, a hypothetical neural circuit model of pair-bond formation (pair-bonding) that includes the ventral tegmental area, nucleus accumbens, paraventricular nucleus, amygdala, hippocampus, anterior olfactory nucleus, and medial prefrontal cortex has been proposed ( Walum and Young, 2018 ). Research implicates oxytocin, vasopressin, dopamine, and, potentially, serotonin and cortisol in pair-bonding ( Walum and Young, 2018 ). Most of these neural regions and endocrine factors have been implicated in romantic love in humans. The implications of this research become apparent when the phylogeny of romantic love is presented.

When applied to romantic love, the second of Tinbergen’s (1963) four questions asks: “How does romantic love develop over the lifetime of an individual?” This can be answered with reference to the age of onset of romantic love, its presence throughout the lifespan, and its duration. Questions of ontogeny also encompass issues around the internal and external influences on romantic love ( Tinbergen, 1963 ; Zeifman, 2001 ). We have also chosen to include some consideration of culture in this section because it influences the causes of romantic love. We find that romantic love first develops in childhood, is experienced at all ages in both sexes, usually lasts months or years, but can exist for many years or decades. It is influenced by a range of internal and external factors and is similar across cultures. The modern environment may influence romantic love in ways not present in our evolutionary history.

Romantic Love Over the Lifetime

Romantic love occurs from childhood through adulthood. It first manifests before puberty ( Hatfield et al., 1988 ), with boys and girls as young as four reporting experiences that equate to romantic love. Adolescence is the time in which romantic love first manifests with all of its characteristic features ( Hatfield and Sprecher, 1986 ), including the onset of sexual desire and activity and, potentially, pair-bonding. Romantic love may be more common among adolescents than young adults. In one study ( Hill et al., 1997 ), American university psychology students reported a greater occurrence of mutual and unrequited love experiences when they were 16–20 years old compared to when they were 21–25 years old. However, romantic love exists at all ages of adulthood in both sexes ( Wang and Nguyen, 1995 ).

There are few studies of psychological sex differences in romantic love. Those that exist (e.g., Hatfield and Sprecher, 1986 ; Hendrick and Hendrick, 1995 ; Cannas Aghedu et al., 2018 ) compare the overall intensity of romantic love and find no difference or slightly more intense romantic love in women than men. To our knowledge, no research has specifically investigated sex differences in duration or form of romantic love although it has been shown that some precursors to romantic love may play a greater role in one sex than the other (see Pines, 2001 ; Sprecher et al., 1994 ; Riela et al., 2010 ). As highlighted above, there are small sex differences in the neurobiology of romantic love ( Bartels and Zeki, 2004 ; Fisher et al., 2006 ) and sex differences may exist in the activity of testosterone ( Marazziti and Canale, 2004 ) and serotonin ( Langeslag et al., 2012 ) in people experiencing romantic love, although findings have been inconsistent. These neurobiological and endocrinological differences may, presumably, have differential effects on the presentation of romantic love which have not yet been identified by research.

The psychological features of romantic love are said to normally last between 18 months to 3 years ( Tennov, 1979 ), although studies have found that serotonin transporter density, cortisol levels, testosterone levels, follicle-stimulating hormone levels, and nerve growth factor levels do not differ from controls 12–24 months after initial measurement ( Marazziti et al., 1999 ; Marazziti and Canale, 2004 ; Emanuele et al., 2006 ). Unrequited love has been shown to last an average duration of between 10 and 17 months, depending on the type of unrequited love ( Bringle et al., 2013 ). In that study, unrequited love for someone that an individual pursued lasted the shortest period of time (10.12 months) and romantic love for someone who an individual knows but has not revealed their love to lasted the longest (18.44 months) in a sample of high school and university students from the United States. This contrasts with reciprocated romantic love that lasted even longer (an average of 21.33 months).

The early stages of romantic love characterized by stress may be distinct from a later period characterized by feelings of safety and calm ( Garcia, 1997 ; de Boer et al., 2012 ). The first stage, which is characterized by approximately the first 6 months of a relationship, has been described as “being in love.” It is marked by all the characteristics of romantic love, including, especially, romantic passion and intimacy. The second phase, which has been said to last from approximately 6 months to 4 years, has been referred to as “passional love.” During this time passion is maintained but commitment and intimacy increase. Passional love gives way to companionate love, passion subsides, and commitment and intimacy reach their peaks. While a description of these phases is informative, it is important to recognize that only one study has investigated these phases and they used a sample of predominately university students ( Garcia, 1997 ). Mechanisms research has not adopted these stages and “early stage” romantic love does not specifically refer to the first 6 months of a romantic relationship.

Romantic love exists on a continuum of intensity but can be classified categorically ( Hatfield and Sprecher, 1986 ). The authors of the Passionate Love Scale ( Hatfield and Sprecher, 2011 ) have developed arbitrary cutoffs for differing intensities of romantic love. However the thresholds that define them are not theoretically or empirically derived and are yet to be widely accepted in the literature.

Romantic love can commence abruptly or build up slowly, although the phenomenon of “love at first sight” may actually be strong attraction rather than romantic love, per se ( Sternberg, 1986 ; Zsok et al., 2017 ). In one study of Chinese and American participants, 38% of participants fell in love fast and 35% fell in love slow, with the remaining unknown ( Riela et al., 2010 ). Another study, of Iranians, found that 70% of participants fell in love slowly or very slowly ( Riela et al., 2017 ). Romantic love can end abruptly but often wanes slowly.

Regardless of the normal duration of romantic love, there is a general inverse relationship between the length of time in a relationship and romantic love ( Hatfield et al., 2008 ; Acevedo and Aron, 2009 ). Romantic love normally gives way to failure of a relationship to form, a relationship breakup, or transition to companionate love. However, in some individuals, romantic love can last many years, or even, decades ( O’Leary et al., 2011 ; Acevedo et al., 2012 ; Sheets, 2013 ). In romantic relationships that last, romantic love serves to bond partners together by creating shared understandings, emotions, and habits ( Hatfield and Walster, 1985 ) characteristic of companionate love and long-term pair-bonds. The transition from romantic love to companionate love is gradual and both types of love share many characteristics. In circumstances where romantic love is maintained beyond the initial few years, obsessive thinking about a partner is no longer a feature (e.g., Acevedo and Aron, 2009 ; O’Leary et al., 2011 ).

Internal and External Influences

A number of internal and external influences affect when, with whom, and how we fall in love. The scenario of attachment, separation, and loss in young children ( Bowlby, 1969 , 1973 , 1980 ) is similar to a “desire for union” and may be the groundwork for romantic attachments in later life ( Hatfield et al., 1988 ). To this extent, romantic love, like newborn/infant attachment, is “prewired” into humans as part of their evolutionary heritage ( Hatfield et al., 1989 ). Researchers “focus their investigations on the effects of mother-infant bonding in order to explain variations in the form, duration, and/or frequency of adult passionate relationships” ( Fisher, 1998 , p. 31). For example, a person’s adult attachment style is determined in part by childhood relationships with parents ( Hazan and Shaver, 1987 ) and this may have implications for the experience of romantic love (e.g., Hendrick and Hendrick, 1989 ; Aron et al., 1998 ). Romantic love is positively associated with a secure attachment style and negatively associated with an avoidant attachment style.

Precursors to romantic love include reciprocal liking, appearance, personality, similarity, social influence, filling needs, arousal, readiness, specific cues, isolation, mysteriousness, and propinquity (see Aron et al., 1989 ; Sprecher et al., 1994 ; Riela et al., 2010 ; Riela et al., 2017 ; see also Hazan and Diamond, 2000 ; Fisher, 2011 ). Research also suggests that conscious variables (personality and appearance), situational variables (proximity and arousal), lover variables (lover finds us attractive, lover fills important needs, similarity, and lover is best friend), and unconscious variables (similarity to relationship with parents, similarity of lover to father, similarity of lover to mother, and love at first sight) contribute to with whom we fall in love ( Pines, 2005 ). The majority of precursors are an interplay between internal and external influences.

Some of the most important precursors to romantic love include personality, reciprocal liking, physical appearance, propinquity, specific cues, readiness, and similarity ( Aron et al., 1989 ; Sprecher et al., 1994 ; Riela et al., 2010 , 2017 ). Personality is the “attractiveness of the other’s personality (e.g., intelligent, humorous)” ( Riela et al., 2010 , p. 474). This represents an interplay between internal influences (the preferences of the individual or what they find attractive) and external influences (the personality characteristics of the potential loved one). Reciprocal liking has been defined above and is a mixture of internal and external influences. Physical appearance, too, is an interplay between what an individual finds attractive, either through genetic predisposition or learned experience, and the physical attributes of the potential loved one. Propinquity has been defined and discussed above and is a combination of internal and external influences. Similarity is “having things in common, including attitudes, experiences, interests, and personal factors such as appearance, personality, and family background ( Riela et al., 2010 , p. 474). This is contingent upon both the individual’s characteristics (internal influence) and the potential loved one’s characteristics (external influence).

There are, however, some precursors that are explicitly internal or external influences. Readiness is “being emotionally or physically prepared for seeking a romantic relationship, such as having just broken up with someone and seeking comfort in a new partner” ( Riela et al., 2010 , p. 475). This can be a largely internal influence that can cause romantic love. Specific cues are “particular characteristics of the other (e.g., smile, shape of the eyes), that are relevant to the perceiver in producing strong attractions. This is not the same as attractiveness in general but refers to highly idiosyncratic features of potential love objects that are specifically important to the individual” ( Riela et al., 2010 , p. 475). These are largely external influences that cause romantic love, although they do trigger a biological or psychological response which is internally determined.

Cross-Cultural Perspectives

There have been a number of books (e.g., Jankowiak, 1995 , 2008 ) and studies that shed light on the cross-cultural nature of romantic love. The sum of research indicates that romantic love is probably universal (although the research is yet to prove this unequivocally) with relatively few psychological differences found between cultures (although cultures respond to love in different ways). An ethnographic analysis of 166 cultures from the Standard Cross-Cultural Sample ( Jankowiak and Fischer, 1992 ; Jankowiak and Paladino, 2008 ) found no evidence of romantic love in only 15 cultures, and this was largely due to lack of data. Validated measures of romantic love (i.e., Passionate Love Scale, Love Attitudes Scale, Triangular Love Scale) have been used in at least 50 countries ( Feybesse and Hatfield, 2019 ). The Triangular Theory of Love is robust cross-culturally ( Sorokowski et al., 2020 ). Cross-cultural accounts of the features and the intensity of romantic love are remarkably similar (see Feybesse and Hatfield, 2019 for a review of cross-cultural perspectives on romantic love). Multiple neuroimaging studies have ascertained that the same neural mechanisms associated with romantic love in American samples are associated with romantic love in Chinese samples ( Xu et al., 2011 , 2012b ).

Romantic love may be thought of more positively among Western countries than other countries and Westerners report falling in love more often (see Feybesse and Hatfield, 2019 ). Cultural differences have also been identified in the role of precursors in causing romantic love. A comparison between Japanese, Russian, and American populations found that culture played a role in the self-reported importance of personality, physical appearance, propinquity, similarity, readiness, isolation, mystery, and social standing ( Sprecher et al., 1994 ). Some differences have also been found between Chinese and Americans ( Riela et al., 2010 ) and between Iranians and Americans ( Riela et al., 2017 ) using similar and different methods. In some cultures, romantic love is suppressed and arranged marriages predominate (discussed below).

Evolutionary Mismatch

The evolutionary mismatch hypothesis argues that humans are now living in environments vastly different from those in which they evolved and, as a result, biological mechanisms may not interact with the environment in the manner that they originally evolved to Li et al. (2018) . Adaptations may malfunction. This has implications for the functioning of mechanisms and psychology. Evolutionary mismatch may influence the occurrence, duration, form, and experience of romantic love. As already suggested, evolutionary mismatch may influence the degree to which certain social mechanisms play a role in causing romantic love. This may have flow-on impacts on the frequency with which an individual falls in love or with whom they fall in love. The increased exposure to potential mates may also lead to greater instances of relationship dissolution and new instances of romantic love than would have been the case in our evolutionary history. Evolutionary mismatch may also influence the duration of romantic love. Under evolutionary conditions, romantic love would usually occur in the context of reproduction, pregnancy, and childbirth (see Goetz et al., 2019 ). This may mean that the duration of romantic love may have been shorter in females than is the case in modern developed societies because they are overcome by mother-infant bonding, possibly at the expense of romantic love.

The form and experience of romantic love may also be impacted by evolutionary mismatch. Technology means that lovers are able to maintain regular contact (e.g., by telephone) or be exposed to images of the loved one (e.g., by photographs) in the absence of physical contact. This consistent exposure may be associated with more frequent activation of neural structures associated with romantic love (i.e., reward and motivation structures) and change the intensity or subjective experience of romantic love compared to evolutionary ancestors who may have been completely separated for periods of time.

Ultimate Perspectives

When applied to romantic love, the third of Tinbergen’s (1963) four questions asks: “What are the fitness-relevant functions of romantic love?” Functional explanations address the fitness ramifications (survival and reproduction) of the behavior or trait of interest ( Tinbergen, 1963 ; Zeifman, 2001 ; Bateson and Laland, 2013 ). We are, thus, concerned with both the fitness-relevant benefits and costs of romantic love. We have outlined the benefits and costs of romantic love associated with five functions based on a small literature on the subject (i.e., Fletcher et al., 2015 ; Buss, 2019 ), reproduction-related literature, and our consideration of the subject. Some of the benefits we describe can be considered functions in their own right (e.g., Buss, 2019 ). Table 3 presents a summary of benefits and costs of romantic love according to five distinct yet interrelated functions: mate choice, courtship, sex, pair-bonding, and health. Our approach is to describe each function, present the benefits associated with each function, and present the costs associated with each function. Where relevant, we have included information about related concepts or theories. We contend that while there is a small amount of evidence for the health promoting benefits of romantic love, the evidence is insufficient to say with certainty that health promotion is a function of romantic love. We conclude this section by summarizing some potential selective pressures and describing romantic love as a complex suite of adaptations and by-products.

Reproduction- and survival-related benefits and costs associated with each function of romantic love.

STI, sexually transmitted infection.

Mate Choice

Romantic love serves a mate choice function ( Fisher et al., 2006 ). Both men and women engage in mate choice ( Stewart-Williams and Thomas, 2013 ). Assessing potential mates has important fitness consequences for individuals, as the benefits of finding a suitable mate are often higher than mating haphazardly or with a randomly selected mate ( Geary et al., 2004 ; Andersson and Simmons, 2006 ; Jones and Ratterman, 2009 ; Shizuka and Hudson, 2020 ). On the other hand, mate choice is a costly and error-prone activity and, thus, it may be adaptive to focus one’s attention on a particular mate that has been identified as a preferred partner ( Bowers et al., 2012 ). Romantic love serves this function.

Mate choice evolved in mammals to enable individuals to conserve their mating energy, choose between potential mates, and focus their attention on particular potential mating partners ( Fisher, 2000 ; Fisher et al., 2006 ). The focus of one’s attention on a single potential mate is not without costs (e.g., Klug, 2018 ; Bear and Rand, 2019 ). Imperfect mate choice (e.g., Johnstone and Earn, 1999 ) could result from imperfect information (e.g., Luttbeg, 2002 ) or acceptance or rejection errors. Imperfect information might include the concealment of information that has detrimental effects on fitness. Time to assess an individual is important in mate choice and imperfect mate choice could potentially be a greater problem in circumstances where romantic love is quick to arise. Mate choice, by definition, excludes other potential mates and romantic love, in fact, suppresses the search for other mates ( Fletcher et al., 2015 ). This cost can be exacerbated in certain environments such as those within which finding additional mates is relatively easy ( Kushnick, 2016 ). Romantic love can detract from other fitness-promoting goals such as career-advancing activities, physical health promoting activities, or forming and maintaining other social relationships.

Romantic love serves a courtship function ( Fisher et al., 2006 , 2016 ). Courtship involves a series of signals and behaviors that serve as a means of assessing potential partner quality and willingness to invest in a relationship ( Trivers, 1972 ; Wachtmeister and Enquist, 2000 ). One function of the attraction system is to pursue potential mates ( Fisher, 2000 ). People in love often engage in courtship of their loved one with the aim of persuading them that they are a good long-term mate.

The primary benefit of courtship in romantic love is that it can secure a mate that is prepared to commit to a relationship. To do this, both sexes can pursue potential mates, display commitment, and signal fidelity ( Buss, 2019 ). These acts are why love has been described as a commitment device ( Frank, 1988 ; Fletcher et al., 2015 ; Buss, 2019 ). Courtship allows individuals to learn about and assess the suitability of potential mates while displaying reproductively relevant resources ( Buss, 2019 ). Men emphasize characteristics such as resources, while women emphasize characteristics such as beauty, in an attempt to increase attractiveness ( Buss, 1988 ; Luoto, 2019a ). Men, at least historically, also provide signals of parental investment ( Buss, 2019 ). Literature on human courtship from an evolutionary perspective supports the notion of greater choosiness among females, predicted by parental investment theory ( Trivers, 1972 ), for short-term mating and less serious commitments. This effect, however, substantially diminishes for long-term mating endeavors and marriage commitment ( Kenrick et al., 1990 ). The literature also suggests that women are looking for specific cues, indicative of evolved preferences, during the courtship process ( Oesch and Miklousic, 2012 ).

There are costs associated with romantic love’s courtship function. These include the expenditure of a significant amount of time and resources and, if courtship efforts are not reciprocated, embarrassment ( Silver et al., 1987 ). Sometimes, individuals in love might engage in intrusive “obsessive pursuit” of someone who is not interested ( Spitzberg and Cupach, 2003 ). Courtship can be a particularly stressful time for an individual. There are also potential costs because individuals who are courting might find themselves in direct intrasexual competition with another individual who has an interest in their potential mate. Intrasexual competition can be costly because an individual must divert additional resources to this endeavor. An individual bears even greater costs if they lose this competition. Both sexes can be subject to costly signaling as part of courtship ( Griskevicius et al., 2007 ), although men are at risk of higher fitness costs associated with temporally extended courtships, despite this being interpreted as a sign of a good mate by women ( Seymour and Sozou, 2009 ).

Romantic love promotes sex and may increase the chances of pregnancy. Sex is an important part of romantic relationships and initiation into sex with a partner, and a greater frequency of sex, is associated with the earlier stages of a romantic relationship ( Call et al., 1995 ). Sex and pregnancy are not, however, features of romantic love in pre-pubescent children and pregnancy is not a feature of romantic love in post-menopausal women. The nature of reproduction is different in societies where contraception and family planning practices are widespread (see Goetz et al., 2019 , for review of evolutionary mismatch in human mating). In such circumstances, immediate pregnancy may not be a feature of romantic love, whereas sex often is. In such circumstances, romantic love may indirectly promote pregnancy by creating pair-bonds whose members later reproduce.

Romantic love provides sexual access ( Buss, 2019 ). Love is one of the most common reasons people give for having sex ( Ozer et al., 2003 ; Meston and Buss, 2007 ; Dawson et al., 2008 ; Meston and Buss, 2009 ). Given the relative willingness of men to engage in short-term mating compared to women, it follows that sex because of love plays a greater role in providing sexual access by women to men than the other way around ( Meston and Buss, 2007 ). Sex can facilitate a gain in reputation ( Meston and Buss, 2007 ) and both sexes increase their status by having children ( Buss et al., 2020 ). Sex is intrinsically pleasurable and reinforcing, and promotes bonding ( Meltzer et al., 2017 ). In times before the advent of contraception, repeated sex with a partner would usually result in pregnancy and childbirth ( Goetz et al., 2019 ; Kushnick, 2019 ). This is still the case in many parts of the world.

For example, there is evidence that features characteristic of romantic love may be associated with a greater number of children among the Hadza, a hunter gatherer tribe in northern Tanzania ( Sorokowski et al., 2017 ). Higher passion, which is definitive of romantic love (e.g., Sternberg, 1986 ), is associated with a greater number of children in women. The findings are important because the lifestyle of the Hadza more closely resembles the environment in which humans evolved than do industrialized or agrarian societies. As a result, inferences can be made about the adaptive function of passion in human evolutionary history. However, intimacy, another component of romantic love ( Sternberg, 1997 ), was found to be negatively correlated with number of children in women. Instead, commitment, a feature of companionate love, was associated with greater number of children in both women and men ( Sorokowski et al., 2017 ). Romantic love is normally relatively short-lived, and therefore the methods used in this study may not have been ideally suited to investigate the fitness consequences of romantic love. Nonetheless, this finding provides some support for the notion that romantic love promotes sexual access by women and facilitates reproduction.

One study ( Sorokowski et al., 2019 ) suggests that romantic love may increase the likelihood of a woman falling pregnant. Higher levels of the gonadotropins, follicle-stimulating hormone, and luteinizing hormone, and a non-significant but positive increase in estradiol to testosterone ratio in women experiencing romantic love could cause increased ovarian activity and increased estradiol synthesis, which might result in higher fecundity ( Sorokowski et al., 2019 ).

The costs associated with romantic love’s sex function are far greater for women than for men ( Trivers, 1972 ). Both sexes could be subject to unwanted pregnancy and associated parenting responsibilities (although this impacts women to a greater extent). There is also, however, a risk of damage to an individual’s reputation. Women are often subject to criticism from other women for engaging in sexual activity ( Koehn and Jonason, 2018 ), especially if a long-term relationship does not result. Men and women risk damage to their reputation for having sex with a low mate value partner, although men are generally treated far more favorably than women for engaging in sexual activity (see Zaikman and Marks, 2017 ). For women, a period of pregnancy followed by a lengthy period of lactation may ensue, and this is costly in terms of the ability to obtain sufficient resources and protecting oneself from harm. There is also the possibility that the relationship will dissolve following pregnancy and the woman may be left to raise a child without the father’s support ( Koehn and Jonason, 2018 ).

Pair-Bonding

Romantic love serves a pair-bonding function ( Fletcher et al., 2015 ). Pair bonding is both a process and a sate characterized by the formation of “enduring, selective attachments between sexual partners” ( Young et al., 2011 , p. 1). It differs from established pair-bonds and the neural characteristics of people experiencing romantic love differ somewhat from what is associated with longer-term pair-bonds (see Acevedo et al., 2012 , for distinction). Evolutionarily, when sex more often led to pregnancy, this pair-bonding would occur in the context of pregnancy and childbirth (although it is unclear if romantic love can exist at the same time as mother-infant bonding). This is still the case in many parts of the world. This is one possible reason for the duration of reciprocated romantic love to be between 18 months and 3 years ( Tennov, 1979 ) when not interrupted by childbirth. The intensity of specific neural activity in people experiencing romantic love is associated with relationship maintenance ( Xu et al., 2012a ).

Romantic love can establish long-term pair-bonds. In both sexes, romantic love promotes the provision of psychological and emotional resources ( Buss, 2019 ) as well as other types of caregiving ( Fletcher et al., 2015 ). It promotes relationship exclusivity through fidelity, jealousy, and mate-guarding ( Buss, 2019 ). Both sexes engage in additional mate retention tactics such as vigilance, mate concealment, monopolization of time, resource display, love and care, or sexual inducements ( Buss et al., 2008 ). Romantic love also promotes the sharing of other resources such as food or money. This benefit for women would have been, and often continues to be, greatest during times of lactation (see Marlowe, 2003 ; Quinlan, 2008 ). Both sexes can also benefit reputationally, as being in a relationship with a high mate value individual confers status, and individuals who are married or in a relationship are viewed more favorably than single people ( DePaulo and Morris, 2006 ). Men experiencing romantic love engage in actions that lead to successful reproductive outcomes ( Buss, 2019 ), such as protecting partners from physical harm. Men also engage in parenting ( Geary et al., 2004 ; Bribiescas et al., 2012 ), which could potentially result in increased offspring survival ( Fletcher et al., 2015 ).

When people are experiencing romantic love they are usually, but not always, interested in pursuing a “long-term mating strategy.” A long-term mating strategy is one that involves commitment, pair-bonding, and the parental investment (if children result) of both partners ( Buss, 2006 ). This contrasts with short-term mating strategies that do not often require public commitment, pair-bonding, and parental investment of the father ( Buss and Schmitt, 1993 ). Pair-bonding is characteristic of a long-term mating strategy.

The concept of romantic love serving as a commitment device is relevant to pair-bonding, as are the concepts of fitness interdependence ( Buss, 2019 ) and self-expansion. Fitness interdependence is the degree to which two people influence each other’s success in replicating their genes ( Aktipis et al., 2018 ). Romantic love binds two individuals together so that the potential reproductive success of one person is contingent upon the success of the other. The self-expansion model suggests that “people seek to expand their potential efficacy to increase their ability to accomplish goals” and that “one way people seek to expand the self is through close relationships, because in a close relationship the other’s resources, perspectives, and identities are experienced, to some extent, as one’s own” ( Aron and Tomlinson, 2019 , p. 2). Fitness interdependence and self-expansion can be increased in people experiencing romantic love.

There are substantial costs associated with pair-bonding ( Kushnick, 2016 ; Klug, 2018 ). Both sexes are potentially missing out on long-term mating opportunities with other suitable mates and are more restricted in terms of short-term mating opportunities ( Geary et al., 2004 ). There is a potential for damage to an individual’s reputation if they are in a relationship with a low mate value individual ( Buss, 2016 ). Both sexes share resources. Pair-bonding is associated with a reduction in the size of an individual’s support network ( Burton-Chellew and Dunbar, 2015 ). Jealousy can have negative effects upon a relationship ( Buss, 2000 , 2019 ; Hatfield et al., 2016 ) and there is a potential for emotional or physical harm arising from a relationship. People sometimes engage in homicide of their current or former partners in response to infidelity, or as a result of jealousy or a breakup ( Buss, 2000 , 2019 ; Shackelford et al., 2003 ). Some women engage in this behavior, but it is predominately a male behavior, when it occurs ( Buss, 2019 ). Stalking can occur following a breakup ( Spitzberg and Cupach, 2003 ; Buss, 2019 ) or, more generally, as a result of romantic love ( Marazziti et al., 2015 ). There is the potential for grief or depression symptoms following the breakup of a relationship ( Verhallen et al., 2019 ). Changing living arrangements, dividing up resources, and legal costs could all be necessary following the dissolution of a pair-bond ( Bear and Rand, 2019 ). Sex-specific costs include sexual obligations to a partner from women and parental investment by men ( Geary et al., 2004 ; Luoto, 2019a ).

While there is evidence that successful pair-bonding is associated with better health and survival ( Fletcher et al., 2015 ), there is little evidence showing that romantic love is associated with good health. Falling in love is associated with alteration in immune cell gene regulation in young women ( Murray et al., 2019 ). Specifically, falling in love is associated with genetic changes that could potentially result in an up-regulation of immune responses to viruses.

Experiencing romantic love for a recently gained partner is associated with the “active/elated” symptoms of hypomania ( Brand et al., 2007 , 2015 ). These symptoms are considered as favorable, “bright side” symptoms and contrast with unfavorable “dark side” symptoms such as disinhibition/stimulation-seeking and irritable/erratic dimensions ( Brand et al., 2015 ). Despite their association with hypomania, the favorable nature of these symptoms in romantic love may be a sign of good physical and mental health because higher hypomanic scores have been associated with higher “mental toughness,” increased physical activity, lower symptoms of depression, and lower sleep complaints ( Jahangard et al., 2017 ). Additionally, falling in love with a partner is sometimes associated with a reduction in depressive symptoms ( Bajoghli et al., 2013 , 2017 ). A reduction in the number of sexual partners could result in a decreased risk of sexually transmitted infections. There is evidence that romantic love might sometimes be associated with improved sleep quality ( Brand et al., 2007 ; Bajoghli et al., 2014 ).

There are some health-related costs associated with romantic love for both sexes. Despite a reduced risk of sexually transmitted infections being a benefit of romantic love, engaging in sexual activity at all may represent an increased risk of sexually transmitted infection, resulting in a cost to some ( Buss, 2016 ; Koehn and Jonason, 2018 ). Infertility from sexually transmitted infections is possible among women ( Koehn and Jonason, 2018 ). Disinhibited/stimulation-seeking and irritable/erratic, depressed, and anxious mood are sometimes features of romantic love ( Wang and Nguyen, 1995 ; Bajoghli et al., 2013 , 2014 , 2017 ; Brand et al., 2015 ; Kuula et al., 2020 ). In the face of repeated unrewarding efforts or adverse events in the courtship process, depressed or anxious mood could result ( Nettle and Bateson, 2012 ). Romantic rejection can result in a major depressive episode or even suicide (see Rantala et al., 2018 ). Despite evidence of improved sleep quality in people experiencing romantic love in some studies ( Brand et al., 2007 ; Bajoghli et al., 2014 ), one study ( Kuula et al., 2020 ) found poorer sleep quality, later sleep timing, and shorter sleep duration (one feature commonly found in studies relied upon to suggest a sleep quality benefit of romantic love) in adolescent girls experiencing romantic love. This suggests that altered sleep may in fact be a detrimental cost in some people experiencing romantic love. Women have the added risk of birth-related complications and death, which has been common in humans until recently in developed countries ( Goldenberg and McClure, 2011 ).

Selective Pressures

The literature contains three interesting theories of possible selective pressures for romantic love. They are framed in the context of promoting the evolution of pair-bonds, but as will be detailed below, the evolution of pair bonds and romantic love are likely to be inexorably linked. All three theories relate to the provision of resources by males to females. The first theory is that pair-bonds and romantic love may have emerged prior to 4 million years ago when bipedalism emerged and hominins moved into the woodlands and savannahs of our ancestral homelands (see Fisher et al., 2016 ). The need for mothers to carry infants in their arms may have driven them to select partners that were wired for pair-bonds which was associated with provisioning, defense, and other forms of support.

The second theory is that biparental care was a driving force in the emergence of long-term mating strategies ( Conroy-Beam et al., 2015 ). A game theoretical approach contends that females selecting males that were wired for pair-bonds directly increased the chances of offspring survival through the provisioning of tangible and intangible resources to the female and offspring. If biparental care was a driving force in the formation of pair-bonds in humans, it would be a uniquely human pressure, as biparental care has been generally identified as a consequence, rather than a cause, of pair-bonds in mammals ( Opie et al., 2013 ; Lukas and Clutton-Brock, 2013 ). This theory also has to contend with the fact that father presence is often not associated with better offspring survival in societies with little access to health care or contraception (see Fletcher et al., 2015 ).

The third theory is that a need for increased fecundity drove the selection of pair-bonds ( Conroy-Beam et al., 2015 ). Periods of malnutrition cause decreased fecundity. Once again, a game theoretical approach suggests that the selection of males that were wired for pair-bonds, which is associated with provisioning of females, increased the caloric intake of females over prolonged periods of time and, in turn, increased fecundity. This hypothesis is appealing because this selective pressure could have been present at any stage among the four hypotheses we propose for the emergence of pair-bonds in a section below.

Romantic Love Is a Complex Suite of Adaptations and By-Products

In evolutionary psychology, an adaptation is “…an inherited and reliably developing characteristic that came into existence as a feature of a species through natural selection because it helped to directly or indirectly facilitate reproduction during the period of its evolution” ( Buss et al., 1998 , p. 535; see also Williams, 2019 ). This approach is based, rightly, on the difficulty of testing hypotheses about the adaptive benefits of traits in ancestral environments. There is an equally valid approach, however, adopted by behavioral ecologists, that views current utility of adaptations as evidence that can be extrapolated to the past ( Fox and Westneat, 2010 ). One definition that has arisen from this approach is that “[a]n adaptation is a phenotypic variant that results in the highest fitness among a specified set of variants in a given environment” ( Reeve and Sherman, 1993 , p. 9).

Taken together, these two approaches to adaptation support the view that romantic love is a “complex suite of adaptations” ( Buss, 2019 , p. 42). The numerous mechanisms recruited in romantic love, the large number of psychological characteristics, and the multiple functions it serves suggest that romantic love may be an amalgamation of numerous adaptations that respond to a variety of adaptive challenges. However, while romantic love may comprise several inter-related adaptations, this does not preclude the possibility that some components are by-products. A by-product is a trait that evolved “not because it was selectively advantageous, but because it was inextricably linked…to another trait that was reproductively advantageous” ( Andrews et al., 2002 , p. 48).

Health-promoting benefits of romantic love, such as elevated mood, increased sleep quality, and up-regulated immune responses, for example, may be by-products of mood circuitry (see Nettle and Bateson, 2012 ; D’Acquisto, 2017 ; Jahangard et al., 2017 ) or other mechanisms, even though they offer some survival or reproductive advantage. Elevated mood, better sleep quality, and an associated up-regulated immune system probably evolved prior to the emergence of romantic love (see Flajnik and Kasahara, 2010 ; Loonen and Ivanova, 2015 ). As a result, it might be prudent to contend that romantic love is a complex suite of adaptations and by-products.

Further, while the evidence points to romantic love as a suite of adaptations and by-products, it is not adaptive in every context. Romantic love continues to have its reproduction-promoting functions in the modern world in some circumstances, either by immediately promoting reproduction, or indirectly promoting reproduction via the formation of romantic relationships, the members of which later reproduce. To that extent, romantic love is sometimes adaptive (see Laland and Brown, 2011 , for distinction between “adaptation” and “adaptive” and lists of benefits, above, for examples of how romantic love can be adaptive). There are circumstances when romantic love may be maladaptive, however, as is evidenced by the substantial fitness-relevant costs of romantic love detailed above. Cogent examples of this are when a loved one is already in a committed relationship or otherwise not interested, when an individual engages in obsessive pursuit that can have social or even legal ramifications, or when violence ensues.

When applied to romantic love, the fourth of Tinbergen’s (1963) four questions asks, “What is the evolutionary history of romantic love?” Phylogenetic explanations focus on the origin and maintenance of a trait in historical evolutionary terms ( Tinbergen, 1963 ; Bateson and Laland, 2013 ). They put a biological trait in a comparative perspective by focusing on the presence or absence of the trait in closely, and sometimes more distantly, related species. In this section, we describe the theory of independent emotion systems and articulate a theory of co-opting mother-infant bonding mechanisms. We examine the primitive structures related to romantic love that arose in our mammalian evolutionary past and were restructured in pair-bonded species. We also examine the particular history of pair-bonds, and thus romantic love, in hominin evolution, with a comparison to other species of primates, especially apes. Finally, we examine the effect of gene-cultural evolutionary issues with regard to romantic love.

Independent Emotion Systems

Fisher’s ( 1998 , 2000 , see also Fisher et al., 2002 ) evolutionary theory of independent emotions systems delineates sex drive (lust), attraction (romantic love), and attachment (pair-bonds). Sex drive is primarily associated with estrogens and androgens and serves to motivate individuals to engage in sexual activity, generally. Attraction is primarily associated with the catecholamines (i.e., dopamine and norepinephrine), phenylethylamine, and serotonin and serves to focus efforts on preferred mating partners. Attachment is primarily associated with oxytocin and vasopressin and serves to enable individuals to engage in positive social behaviors and connections of a sufficient length of time to satisfy species-specific parenting approaches ( Fisher, 1998 ). Sex drive relates most to the sex function of romantic love, attraction to the mate choice and courtship functions, and attachment to the pair-bonding function. Romantic love shares similarities with the ‘courtship attraction system’ found in many mammals ( Fisher et al., 2006 ).

Co-opting Mother Infant Bonding Mechanisms

While the theory of independent emotion systems ( Fisher, 1998 , 2000 ; Fisher et al., 2002 ) has been the predominate theoretical account of the evolution of romantic love for more 20 years, comparative studies, imaging studies, and assessments of psychological characteristics have raised the possibility of a complimentary evolutionary theory, that of co-opting mother-infant bonding mechanisms. Literature on romantic love, maternal love (of which mother-infant bonding is a part), mother–infant bonding, and pair-bonding ( Bartels and Zeki, 2004 ; Ortigue et al., 2010 ; Numan and Young, 2016 ; Walum and Young, 2018 ) suggests romantic love may have evolved by co-opting mother-infant bonding mechanisms. Co-option is an evolutionary process whereby a trait (e.g., mechanism, morphology, behavior) is repurposed – that is, it serves a different function to that which it originally served (see McLennan, 2008 ).

Animal research, focusing on mammals, and involving, monogamous prairie voles, finds substantial similarities between mother-infant bonding mechanisms and pair-bonding mechanisms ( Numan and Young, 2016 ). “[A]mygdala and nucleus accumbens–ventral pallidum (NA–VP) circuits are involved in both types of bond formation, and dopamine and oxytocin actions within NA appear to promote the synaptic plasticity that allows either infant or mating partner stimuli to persistently activate NA–VP attraction circuits, leading to an enduring social attraction and bonding” ( Numan and Young, 2016 , p. 98). Some of these circuits do not appear to be involved in human romantic love, but there are other similarities that support a theory of co-opting mother-infant bonding in humans.

Several brain regions implicated in romantic love overlap precisely with that involved in maternal love. This includes activity in numerous regions that are associated with a high density of oxytocin and vasopressin receptors ( Bartels and Zeki, 2000 , 2004 ) although it should be noted that in the study that asserts this, participants included mothers experiencing maternal love beyond the mother-infant bonding stage. A meta-analysis of love also found romantic and maternal love shared activity in dopamine-rich areas ( Ortigue et al., 2010 ). Almost nothing is known about the mechanisms regulating the infant side of mother-infant bonding. However, some inferences have been made from animal models which suggest that the mechanisms may be similar to those regulating the maternal side, but without involvement of the amygdala (see Sullivan et al., 2011 , for review).

There are substantial psychological similarities between romantic love and early parental love, of which mother–infant bonding is a part. Extreme similarities exist between romantic love and early parental love in the domains of altered mental state, longing for reciprocity, idealization of the other, and dichotomous resolution of the establishment of intimate mutually satisfying reciprocal patterns of interaction usually marked by a culturally defined ritual ( Leckman and Mayes, 1999 ). Similar trajectories of preoccupation in romantic love and parental love also exist. In romantic love, preoccupation increases through the courtship phase and peaks at the point of reciprocity where preoccupation begins to slowly diminish. In parental love, preoccupation increases throughout pregnancy and peaks at the point of birth where preoccupation begins to diminish.

Mammalian Antecedents

Romantic love in humans is caused by physiological mechanisms whose evolutionary roots were planted in our early mammalian ancestors. These evolutionary roots provided the raw materials that were fleshed out, in evolutionary time, to form the basis of a wide range of social behaviors in mammals, including those related to sex drive, mate choice, and attachment ( Fisher, 1998 , 2000 ; Fisher et al., 2002 ; Broad et al., 2006 ; Carter and Perkeybile, 2018 ; Curley and Keverne, 2005 ; Fisher et al., 2006 ; Fischer et al., 2019 ; Johnson and Young, 2015 ; Numan and Young, 2016 ; Porges, 1998 ). Romantic love may have evolved after the neural circuitry associated with mate choice became populated by oxytocin receptors which played a role in the evolution of enduring social attraction and pair-bond formation (see Numan and Young, 2016 ). “[P]air bonding is the evolutionary antecedent of romantic love and…the pair bond is an essential element of romantic love” ( Walum and Young, 2018 , p. 12).

Examining the similarities between the neurobiological and endocrinological mechanisms involved in mother-infant bonding and pair-bonding in mammals, it becomes apparent that the maternal functions of this suite of adaptations arose deep in the evolutionary history of mammals ( Numan and Young, 2016 ). Their derived, pair-bonding functions would have arisen later in a very small number of species (only 3–5% pair-bond). As such, the neural circuitry and other proximate mechanisms involved in mother-infant bonding in mammals “may have provided a primordial neural scaffold upon which other types of strong social bonds, such as pair bonds, have been built” ( Numan and Young, 2016 , p. 99). We are, thus, on reasonably solid ground to posit evolutionary trajectories of romantic love. Figure 1 presents information and hypotheses about the evolutionary history of romantic love. Evolutionary trajectories of romantic love start with the ancestral mammalian mother–infant bonding mechanisms and culminate in their co-option and modification for pair-bonding in several mammalian lineages ( Numan and Young, 2016 ). Human romantic love results from one of these trajectories. In another trajectory—the one that includes pair-bonding prairie voles ( Microtus ochrogaster )—we know quite a lot about the functioning of oxytocin, vasopressin, and dopamine in facilitating pair-bonding (e.g., Carter and Getz, 1993 ; Carter and Perkeybile, 2018 ; Walum and Young, 2018 ). Although these derived changes to the primitive mammalian machinery may not be the direct evolutionary antecedents of those at work in humans (they are, rather, the product of parallel evolution), they provide a window into how basic machinery can be modified to affect those ends. One substantive difference appears to be the relative importance of the hormonal drivers in the smaller species, and the dopamine-related ones in humans ( Broad et al., 2006 ; Fisher et al., 2016 ).

Phylogenetic relationships among select mammal species that pair-bond.

Pair-Bonds in Primates

Humans are members of the primate superfamily Anthropoidea, amongst whom there is great diversity in social systems, and whose ancestral state likely included complex group-based social relationships ( Kay et al., 1997 ; Shultz and Dunbar, 2007 ). This would have included long-term association between unrelated males and females—which is a far cry from the solitary system that is modal and ancestral for mammals ( Lukas and Clutton-Brock, 2013 ; Opie et al., 2013 ). There are even some members of this lineage who have evolved pair-bonds, such as the marmosets and tamarins (Callitrichidae), and gibbons (Hylobatidae). The similarities between these species and humans in terms of the adaptive suite related to pair-bonds, like the similarities between humans and voles, are due to convergent/parallel evolution ( French et al., 2018 ).

Our closest living relatives are the common chimpanzee ( Pan troglodytes ) and bonobo ( Pan paniscus ) with whom we share a common ancestor just 5–8 million years ago. While bonobos are alluring due to their free-willed sexual nature, common chimpanzees provide a better glimpse into the behavior of our direct ancestors. Although the common chimpanzee mating system is defined as promiscuous, there are, in fact, three forms of common chimpanzee mating tactics ( Morin, 1993 ). The first two—possessive mating and consortships—involve some of the characteristics we associate with romantic love, such as a more-than-fleeting association and mate guarding, but they are much rarer than the third type, opportunistic mating. The comparison of chimpanzees and humans, thus, suggests that one possible hypothesis for the emergence of romantic love is that it originated in their common ancestor (H1 in Figure 1 ). Alternatively, it might be that the common ancestor had an adaptive repertoire that was primed for its emergence when the requisite socioecological context arose. In this way, the evolution of romantic love from chimp-like mating is similar to the evolution of human culture from chimp-like culture.

For some, the origin of romantic love was more likely to have fallen somewhere on our side of the human–chimpanzee split (e.g., Fisher et al., 2016 ). Even so, we are left with the difficulty of pinpointing exactly when it arose—attributable to there being only one extant hominin species from amongst the many that have existed ( Pigliucci and Kaplan, 2006 ) and the lack of direct fossil evidence for romantic love. If we accept the conventional view that romantic love evolved to facilitate pair-bonding, then we can search for clues about the evolution of the former by tracing the evolution of the latter ( Fletcher et al., 2015 ). A transition from ape-like to human-like sexual behavior in our lineage may have pre-dated the emergence of the genus Homo ( Lovejoy, 1981 )—and, thus, we have a second hypothesis (H2 in Figure 1 ). A comparison of sexual dimorphism in Australopithecus and early genus Homo , however, suggests a third hypothesis—that it arose after their emergence (H3 in Figure 1 ). Several lines of evidence suggest that the earliest members of our species, Homo sapiens , pair-bonded but were not necessarily monogamous. Based on an examination of the distribution of mating systems in modern, small-scale human societies and three correlates of primate mating systems ( Dixson, 2009 ), it is possible to conclude that pair-bonds are a “ubiquitous” feature of human mating that can manifest through polygyny or polyandry, but most commonly occur in the form of serial monogamy ( Schacht and Kramer, 2019 ). The final hypothesis, thus, is that romantic love is the unique domain of our species (H4 in Figure 1 ).

The transition to mostly monogamous and some polygynous groupings could have had a transitional phase where polygynous groupings were the norm ( Chapais, 2008 , 2013 ). Pair-bonds may have arisen from a complex interaction between the fitness benefits and costs of mating and parental care ( Quinlan, 2008 ). The transition from ape-like promiscuity to human pair-bonds may have been driven by the provision of females by low-ranking males ( Gavrilets, 2012 ). The direct benefits for females was the food provided, for the males, the mating opportunity. This may have led to selection for males that were less aggressive and more prosocial. The female mate-choice mechanism is a distinct possibility for explaining human self-domestication ( Gleeson and Kushnick, 2018 ).

Gene-Culture Coevolution

Romantic love is a universal or near-universal feature in human societies ( Jankowiak and Fischer, 1992 ; Gottschall and Nordlund, 2006 ; Jankowiak and Paladino, 2008 ; Fletcher et al., 2015 ; Buss, 2019 ; Sorokowski et al., 2020 ). A small number of genetic correlation studies show that there are a number of genes associated with romantic love ( Emanuele et al., 2007 ; Murray et al., 2019 ; Acevedo et al., 2020 ). Other insights into the genetic evolution of romantic love can be garnered from elsewhere, however. For example, life history theory provides insight into ethnic or geographical variation in romantic love and its role in providing sexual access by women.

Romantic love is among the most common reasons female adolescents give for having sex ( Ozer et al., 2003 ). A “slow” life history strategy is associated with eros more than other loving styles ( Marzec and Łukasik, 2017 ). Psychopathology associated with impulsivity is a feature of a “fast” life history strategy, as is promiscuous sexuality ( Del Giudice, 2016 ). Greater impulsivity is associated with a reduced likelihood of giving romantic love as a reason for having sex among adolescent females ( Dawson et al., 2008 ).

As a result, genetic determinants of life history strategies (e.g., Figueredo et al., 2004 ) may influence the occurrence of romantic love. National scores on the life history strategy genetic factor index correlate with adolescent fertility rates indicating that genetic predictors of a fast life history are associated with higher rates of adolescent pregnancy ( Luoto, 2019b ). This ethnic or geographical variation in the genetic determinants of life history strategies may also represent ethnic or geographic variation in the genetic determinants and reproductive relevance of romantic love.

In addition to this, cultural factors may have affected the role of romantic love in mating and marriage decisions—and this has implications for understanding the evolution of romantic love ( Fletcher et al., 2015 ). Arranged marriages are the norm in 80% of 200 forager societies from the Ethnographic Atlas ( Apostolou, 2007 ). Phylogenetic methods to reconstruct the ancestral marriage patterns of our species using the same data found that there were likely marriage transactions (brideprice or brideservice) but only a limited amount of polygyny ( Walker et al., 2011 ). While the ancestral state for arranged marriages was not definitive, arranged marriages were likely present around 50 thousand years ago, when our ancestors expanded their range beyond Africa. So, despite romantic love being viewed as an important component of marriage and mating, it may have played a role of decreasing importance in the recent evolutionary history of our species. Arranged marriages may have limited the role of female mate choice in intersexual selection ( Apostolou, 2007 ). Further, despite romantic love’s decreased role in courtship and marriage, it may have continued to serve a role in facilitating pair-bonding as romantic love can develop even in the arranged-marriage context. The role of romantic love in facilitating mate choice, courtship, and marriage may now be increasing with the decline and modification of arranged marriages in many parts of the world (e.g., Allendorf and Pandian, 2016 ). This may be the result of the increasing sexual equality of women (e.g., de Munck and Korotayev, 1999 ).

Romantic love is a complex and multifaceted aspect of human biology and psychology. Our approach in this review has been to highlight how Tinbergen’s (1963) “four questions” can help us to synthesize the important strands related to the mechanisms, development, fitness-relevant functions, and evolutionary history of this phenomenon. Here, we synthesize what this review has presented in each level of explanation and suggest what this indicates about other levels of explanation. We then highlight some gaps in our knowledge that could be filled with future research and present a new ethologically informed working definition of romantic love.

What Do Tinbergen’s Four Questions Tell Us?

One of the benefits of using Tinbergen’s four questions as a framework to describe a complex trait such as romantic love is its ability for one level of explanation to provide insights into the other level of explanation (see Tinbergen, 1963 ; Bateson and Laland, 2013 ; Zietsch et al., 2020 ). In particular, an understanding of the proximate causes of romantic love has provided insights into the functions and phylogeny of romantic love although an understanding of the ultimate level of explanation provides some insights into the mechanisms of romantic love.

Multiple mechanistic systems involved in romantic love suggests it may serve multiple functions and may be a suite of adaptations and by-products rather than a single adaptation. We found that romantic love is associated with activity in a number of neural systems: reward and motivation, emotions, sexual desire and arousal, and social cognition. It is also associated with activity in higher-order cortical brain areas that are involved in attention, memory, mental associations, and self-representation. We also found that romantic love is associated with a number of endocrine systems: sex hormones, serotonin, dopamine, oxytocin, cortisol, and nerve growth factor. This is consistent with our position that romantic love serves mate choice, courtship, sex, and pair-bonding functions. Reward and motivation system activity may be particularly involved in the mate choice function of romantic love. Cortisol may be particularly indicative of the courtship function of romantic love, which overlaps with pair-bonding. Neural areas associated with sexual desire and arousal and the activity of sex hormones may play a particular role in the sex function. Finally, reward and motivation regions of the brain (rich with oxytocin receptors) and activity of the oxytocin system may play a particular role in the pair-bonding function of romantic love. Our understanding of the biological mechanisms that cause romantic love supports our description of romantic love’s functions.

Mechanistic similarities between romantic love and mother-infant bonding suggest that romantic love may have evolved by co-opting mother-infant bonding mechanisms. This articulates one hypothesis about the evolutionary history of romantic love that complements the predominate theory of independent emotion systems ( Fisher, 1998 , 2000 ; Fisher et al., 2002 ). This is supported by the psychological similarities between romantic love and early parental love.

Evidence of substantial activity of oxytocin receptor rich brain regions and the oxytocin endocrine system in romantic love lends weight to the position that romantic love only evolved after the neural circuitry associated with mate choice, specifically, regions of the mesolimbic reward pathway and dopamine rich areas, became populated by oxytocin receptors specifically receptive to stimuli from mating partners. That played a role in the evolution of enduring social attraction and pair-bond formation ( Numan and Young, 2016 ). This supports our claim that romantic love probably evolved in conjunction with pair-bonds in humans. As a result, we are bolstered when we contend that romantic love emerged relatively recently in the history of humans.

The duration of romantic love also raises questions about the functions of romantic love. It has been said that the psychological features of romantic love can last from 18 months to 3 years in reciprocated romantic love. However, in our evolutionary history, romantic love would have usually occurred in the context of pregnancy and child birth. Mother-infant bonding becomes active around the time of childbirth. We are not aware of any research that has investigated whether romantic love can occur at the same time as mother-infant bonding or whether it must subside for mother-infant bonding to become active. Answering this question would elucidate if the functions of romantic love extinguish once reproduction has been successful. The existence of long-term romantic love also raises questions about the functions of romantic love. It has been posited that long-term romantic love is “part of a broad mammalian strategy for reproduction and long-term attachment” ( Acevedo et al., 2020 , p. 1). This may indicate that long-term romantic love serves similar functions to romantic love that lasts a shorter period of time.

Just as the multiple biological mechanisms involved in romantic love suggests a variety of functions, the functions of romantic love specified in our review suggests specific biological mechanisms are involved. As outlined above, specific functions may be associated with specific mechanisms and this should be an area of targeted research.

The possibility of romantic love evolving by co-opting mother-infant bonding mechanisms raises a number of possibilities in relation to the proximate causes of romantic love. It suggests that social activity associated with mother–infant bonding (e.g., filling of needs, specific cues) may be particularly important precursors to, or features of, romantic love. It suggests that many of the genes and polymorphisms involved in causing romantic love may have been present in mammals since the emergence of mother–infant bonding, making comparative animal research using mammals relevant. It also suggests that further research into shared neural activity between romantic love and mother–infant bonding is warranted.

We contend that romantic love probably emerged in conjunction with pair-bonds in humans or human ancestors. As such, further information about the similarities and differences between romantic love (pair-bonding) and companionate love (established pair-bonds) is needed. In particular, information about any role of the mesolimbic pathway (see Loth and Donaldson, 2021 ) or regions associated with sexual desire in companionate love would help to shed light on the evolutionary history of pair-bonding and pair-bonds. Specifically, this could shed light on if, as has been suggested (see Walum and Young, 2018 ), romantic love and pair-bonds are inextricably linked.

Areas of Future Research

One issue with research into the mechanisms of romantic love is that it has, with some exceptions (e.g., Fisher et al., 2010 ), utilized samples of people experiencing romantic love who are in a relationship with their loved one. Romantic love serves a mate choice and courtship function, and as a result, a large proportion of people experiencing romantic love are not in a relationship with their loved one (e.g., Bringle et al., 2013 ). A small number of studies have directly investigated unrequited love (e.g., Tennov, 1979 ; Baumeister et al., 1993 ; Hill et al., 1997 ; Aron et al., 1998 ; Bringle et al., 2013 ), but none of these investigated the mechanisms that cause romantic love. Studying such people might identify the specific contributions of particular mechanisms to particular functions. For example, the mechanisms associated with the pair-bonding function of romantic love may not be active in individuals who are engaging in courtship and the mechanisms involved in courtship may not be present in lovers who are already in a relationship with their loved one. Research would benefit from considering the mechanisms that underlie related psychopathologies and it would be useful to understand the relationship between mate preferences and romantic love.

Molecular genetics research, such as that undertaken by Acevedo et al. (2020) , could further identify contributions of genes in people experiencing romantic love. Resting state fMRI provide an opportunity to investigate networks characteristic of psychopathology related to romantic love. Research should investigate the automatic/internal emotional regulatory network and the volitional/external regulatory network associated with mania/hypomania in people experiencing romantic love. Further research is required into the endocrinology of romantic love. In particular, further research is needed into the role of opioids, corticotropin-releasing factor, glutamate, acetylcholine, and vasopressin in romantic love. Efforts should be made to combine psychological and mechanisms research. For example, differences in neural or endocrine activity may be present in people experiencing romantic love who display elevated symptoms of depression compared to those who display reduced symptoms. As a result, neuroimaging and endocrinological studies could categorize people experiencing romantic love according to their levels of depression or type of hypomanic symptoms.

Given the large number of fMRI studies, interpreting the neuroimaging literature can be overwhelming. It has been nearly 10 years since the last meta-analysis of fMRI studies including romantic love. It is time for another one that focuses solely on romantic love. There is also a pressing need to attempt to replicate and extend endocrine studies and to specifically investigate the oxytocin system in people experiencing romantic love using validated measures of romantic love. As with many areas of psychological research ( Henrich et al., 2010 ), and specifically in areas related to mating psychology ( Apicella et al., 2019 ; Scelza et al., 2020 ), there is a pressing need to ensure that samples used in research are not exclusively Western, educated, industrialized, rich, and democratic.

Limited ontogeny research has elucidated the mechanisms causing romantic love across the lifespan. The literature that has (e.g., Luoto, 2019a ), has focused on mate choice, rather than romantic love, per se . We know nothing about the neurobiology or endocrinology of romantic love in children or about the endocrinology of long-term romantic love. It would be useful to investigate how the functions of romantic love differ according to age of individuals or the duration of romantic love. Internal and external factors influence romantic love, although there has been surprisingly little research into this topic. It would be prudent to continue to develop a more detailed understanding of the factors that lead to romantic love (e.g., Riela et al., 2010 , 2017 ). It would be useful to better understand the relationship between attachment styles and romantic love. Research should investigate if romantic love can occur at the same time as mother-infant bonding, or if they are mutually exclusive states.

Research into the functions of romantic love is sparse. There is a need for clear, evidence-informed definitions and descriptions of each of the functions of romantic love. It is likely that different mechanisms moderate different functions, and research should attempt to determine the contribution of specific genetic, neural, and endocrine activity to each individual function (see Zietsch et al., 2020 ). The advent of contraception and the adoption of family planning strategies means romantic love now serves more of a sex function than a pregnancy function in some environments. This is particularly the case early in a relationship. Pregnancy may become a feature as a relationship progresses and the fitness consequences of romantic love need to be investigated. Romantic love’s role as a suite of adaptations and by-products should be investigated. There is theoretical support for the notion that romantic love serves a health-promoting function (e.g., Esch and Stefano, 2005 ); however, there is a limited number of studies demonstrating a health-promoting effect of romantic love.

The relative infancy of genetic research, the lack of a clear fossil record, and the small number of species with which comparative analysis can be undertaken, means novel and creative means of investigating the phylogeny of romantic love must be undertaken. There is a need to pin-point the phylogenetic emergence of romantic love and the factors that caused it. To do this, more research into the genetics of romantic love must be conducted, and this should consider the phylogeny of specific genes and polymorphisms (e.g., Acevedo et al., 2020 ; see also Walum and Young, 2018 ). Efforts to assess the contribution of sexual selection to the evolution of romantic love are warranted. Studies of newly discovered fossils can help to identify shifts in sexual dimorphism that are indicative of pair-bonds. Further observational and experimental research into romantic love in hunter-gatherer tribes could tell us more about how romantic love functioned in our evolutionary history. Comparative research still has much to contribute. Research should explore the possibility that initial changes to the ancestral mammalian physiology that led directly to human romantic love arose in response to selection on both mating and non-mating-related behavior, such as pro-sociality (e.g., Barron and Hare, 2020 ; Luoto, 2020 ) or unique aspects of our species’ parenting repertoire. It might be fruitful to further investigate the relationship between romantic love and life history theory (e.g., Olderbak and Figueredo, 2009 ; Marzec and Łukasik, 2017 ). Finally, efforts should be made to elaborate and test the theory that romantic love emerged by co-opting mother–infant bonding mechanisms.

A New Working Definition of Romantic Love

The introduction to this review provided four definitions or descriptions of romantic love. For decades, most definitions ( Hendrick and Hendrick, 1986 ; Sternberg, 1986 ; Hatfield and Rapson, 1993 ) of romantic love have informed research into the cognitive, emotional, and behavioral characteristics of romantic love. The past two decades, however, have seen an increasing focus on the biology of romantic love. Only recently has an evolution-informed definition been proposed ( Fletcher et al., 2015 ). That working definition, however, does not incorporate much of the research that provides insight into the proximate and ultimate causes of romantic love.

We believe that the analytical approach taken in this review has identified sufficient information to justify the development of a new ethologically informed working definition of romantic love. The purpose would be to create an inclusive definition that is useful for researchers in varied disciplines investigating romantic love’s psychological characteristics, genetics, neurobiology, endocrinology, development, fitness-relevant functions, and evolutionary history. It may also be of use to psychologists and psychiatrists attempting to understand the experience and etiology of romantic love in their practice. It should be sufficiently precise and descriptive to both guide and link research. We provide, here, a working definition of romantic love:

Romantic love is a motivational state typically associated with a desire for long-term mating with a particular individual. It occurs across the lifespan and is associated with distinctive cognitive, emotional, behavioral, social, genetic, neural, and endocrine activity in both sexes. Throughout much of the life course, it serves mate choice, courtship, sex, and pair-bonding functions. It is a suite of adaptations and by-products that arose sometime during the recent evolutionary history of humans.

We situate the study of romantic love within the context of existing human mating literature. Our definition recognizes that romantic love is experienced across the lifetime of an individual, that research has shed light on the social, psychological, genetic, neural, and endocrine characteristics associated with it, and that it occurs in both sexes. Our definition also recognizes that romantic love serves a variety of functions and that these functions may vary across the lifespan. It does not exclude long-term or unrequited romantic love from the definition. Health is not identified as a function of romantic love in our definition despite being considered in our review. If more evidence comes to light, this definition can be amended to incorporate health.

Our definition has similarities and differences with the definition proposed by Fletcher et al. (2015) . This is appropriate given both are informed by evolutionary approaches which differ somewhat. We do not specifically define romantic love as being a commitment device or reference passion, intimacy, and caregiving. In our review, we recognize that romantic love is a commitment device and serves to display commitment and signal fidelity as part of its courtship function. We believe that reference to romantic love’s behavioral activity and courtship and pair-bonding functions sufficiently encapsulate this concept. Sternberg’s (1997) definition of romantic love and Fletcher et al.’s (2015) definition include references to passion and intimacy. Caregiving (e.g., provision of psychological and emotional resources, sharing resources), while associated with pair-bonding, is not sufficiently definitive of romantic love using Tinbergen’s four questions as a framework to include in our definition.

We do not reference the universality of romantic love. While some experts assert its universality (e.g., Fletcher et al., 2015 ; Buss, 2019 ), we believe that the finding of Jankowiak and Fischer (1992) leaves enough uncertainty for it to be prudent to omit this aspect from our definition. Their research has found no evidence of romantic love in fifteen cultures (see Jankowiak and Paladino, 2008 , for update to the original investigation) although this is probably the result of lack of data rather than evidence to the contrary. Once this matter is settled, which could be achieved by further investigating those societies where no evidence of romantic love was found, the definition can be amended. Fletcher et al. (2015) state that romantic love is associated with pair-bonds. We do the same by stating that pair-bonding is one of the functions of romantic love.

We also do not make specific reference to romantic love suppressing the search for mates. We recognize this as a cost in our review, but do not believe that this is so definitive of romantic love to include in our definition. Rather, we believe that our reference to “behavioral” activity and the “mate choice” function of romantic love in our definition sufficiently accommodates this feature. Our definition provides more detail than that provided by Fletcher et al. (2015) by including elements derived from substantial research into the mechanisms, ontogeny, functions, and phylogeny of romantic love. Like the Fletcher et al. (2015) definition, our definition recognizes that romantic love has distinct psychological characteristics and that we know about some of the proximate mechanisms that regulate it. However, as explained above, we do not include reference to the health-promoting effects of romantic love.

As more information about romantic love is gathered, we anticipate the definition to develop. However, we believe that this definition is an improvement upon previous definitions and adequately captures what is currently known about romantic love’s proximate and ultimate causes. It would be useful for researchers investigating romantic love from myriad perspectives. This definition should be critiqued and improved, and we welcome any such efforts from researchers and theorists across the spectrum of academic disciplines.

Our review provides a comprehensive account of the phenomenon known as romantic love. It covers topics such as social precipitants, psychology, genetics, neurobiology, and endocrinology. It provides an account of romantic love across the lifetime of an individual and is the first to propose four discrete reproduction-related functions of romantic love supported in the literature: mate choice, courtship, sex, and pair-bonding. It provides a summary of the benefits and costs of romantic love, outlines possible selective pressures, and posits that it is a complex suite of adaptations and by-products. We propose four potential evolutionary histories of romantic love and introduce the theory of co-opting mother-infant bonding mechanisms. We have identified a number of specific and general areas for future research. Our review suggests a new, ethologically informed working definition of romantic love that synthesizes a broad range of research. The working definition we propose serves to define a complex trait in a way that can both guide and link research from a variety of fields.

Author Contributions

AB conceived the manuscript. AB and GK collaborated on the development of the analytical framework and writing of the manuscript. Both authors approved the final version.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We would like to thank the reviewers for comments that helped to improve the manuscript.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fpsyg.2021.573123/full#supplementary-material

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The origin and evolution of love

Senior Lecturer in Evolutionary Biology and Paleontology, University of Bath

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Nicholas R. Longrich does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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Why do we love? At best, it’s a mixed blessing, at worst, a curse. Love makes otherwise intelligent people act like fools; it causes heartache and grief. Lovers break our hearts, family sometimes drive us mad, friends can let us down.

But we’re hard-wired to bond with each other. That suggests the capacity for love evolved, that natural selection favoured caring for one another. Fossils tell us that love evolved hundreds of millions of years ago, helping our mammalian ancestors survive in the time of the dinosaurs.

Humans have peculiarly complex emotional lives. Romantic love, the long-term bonding between males and females, is unusual among mammals. We’re also unusual in forming long-term relationships with unrelated individuals (friendships).

But humans and all other mammals share one kind of love, the bond between a mother and her offspring. The universality of this attachment suggests that it’s the original, ancestral form of bonding – the first kind of love, from which all others evolved.

Evidence of parent-offspring bonding appears around 200 million years ago, in the latest Triassic and earliest Jurassic periods. Fossils of Kayentatherium , a Jurassic proto-mammal from Arizona, preserve a mother who died protecting her 38 tiny babies . For this behaviour to exist, the instincts of both mother and offspring first had to evolve.

In primitive animals, such as lizards, parents aren’t exactly parental. A mother Komodo dragon abandons her eggs, leaving hatchlings to fend for themselves . If she ever meets her young, she is likely to try to eat them: komodo dragons are cannibals . The young will instinctively run for their lives upon meeting her – and should.

Guarding hatchlings requires that the mother evolve instincts to see her small, helpless offspring as things to protect, not easy prey. Meanwhile, the offspring must evolve to see mum as a source of security and warmth, not fear.

Kayentatherium ‘s fossilised mother-child association implies that this instinctive evolution had already happened. But Kayentatherium probably wasn’t a doting mother. With 38 kids, she probably couldn’t feed them, or spend much time on them.

In Welsh rocks laid down in the Late Triassic, we find evidence of more advanced parental care. Here, the proto-mammal Morganucodon shows mammal-style tooth replacement . Instead of endlessly replacing teeth from birth to death –as in lizards and sharks – Morganucodon was toothless as a baby, developed baby teeth, then shed those for adult teeth.

This replacement pattern is associated with lactation. Babies that suckle milk don’t need teeth. So Morganucodon mothers made milk. Providing more care to her young, Morganucodon probably invested heavily in a few offspring, like modern mammals, and would have evolved a correspondingly stronger bond with them. The young, completely dependent on the mother for food, would have developed a stronger emotional attachment as well.

It’s at this point in mammal history that our mammal ancestors stopped seeing each other as lizards did, exclusively in terms of danger, food and sex, feeling only the primitive emotions of fear, hunger and lust. Instead, they started to care for one another. Over millions of years, they increasingly began to bond, to protect and seek protection, exchange bodily warmth, groom one another, to play with, teach and learn from each other.

Mammals evolved the ability to form relationships. Once they did, this adaptation could be used in other contexts. Mammals could form relationships as family and friends in sophisticated social groups: elephant herds , monkey troops, killer whale pods , dog packs , human tribes. And in some species, males and females formed pair bonds.

Romantic love between men and women is a recent evolutionary development, associated with males helping females care for children. In most mammals, males are absentee fathers, contributing genes and nothing else to their offspring. In our closest relatives, chimpanzees, paternal care is minimal .

In a few species, including beavers , wolves, some bats, some voles and Homo sapiens , pairs form long-term bonds to cooperatively raise children. Pair bonding evolved sometime after our ancestors split from chimps, 6 million to 7 million years ago – probably before the split between humans and Neanderthals.

Love in our DNA

We can guess that Neanderthals formed long-term relationships, because their DNA is in us . That implies that Neanderthals and humans didn’t simply mate. We had children, who themselves became parents, and grandparents, and so on. For the results of those unions to not just survive, but thrive and integrate into their tribe, mixed children were likely to have been born to parents who cared for them – and each other.

Not all encounters between our species were peaceful or pretty , but neither were they entirely violent. Neanderthals were different from Homo sapiens , but enough like us that we could love them, and they, us – even coming from different tribes. A love story worthy of Jane Austen, literally written into our species’ DNA.

There’s an adaptive benefit to love. Today, the ecosystem is dominated by animals with parental care. Mammals and birds, and social insects including ants, wasps, bees and termites, all of which care for their young, dominate terrestrial ecosystems. Humans are the dominant terrestrial animal on Earth .

Parental care is adaptive by itself, but by teaching animals to form relationships, it also paved the way for the evolution of sociality and cooperation on a larger scale. Parental care in wood roaches, for example, led one lineage, termites, to evolve vast family groups (colonies) that literally reshape the landscape .

Ants, forming up to 25% of the biomass of some habitats , likely evolved coloniality in the same way. Evolution can be violently competitive, but the ability to care and form relationships allowed for cooperative groups, which became effective competitors against other groups and species.

Caring helps us cooperate and cooperation helps us compete. Humans can be selfish and destructive. But we’ve dominated the planet only because an unparalleled ability to care for one another – for partners, children, families, friends, fellow humans – allowed cooperation on a scale never before seen in the history of life.

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Professor in Physiotherapy

Postdoctoral Research Associate

HYPOTHESIS AND THEORY article

Towards a comprehensive theory of love: the quadruple theory.

• Independent Researcher, San Diego, CA, United States

Scholars across an array of disciplines including social psychologists have been trying to explain the meaning of love for over a century but its polysemous nature has made it difficult to fully understand. In this paper, a quadruple framework of attraction, resonance or connection, trust, and respect are proposed to explain the meaning of love. The framework is used to explain how love grows and dies and to describe brand love, romantic love, and parental love. The synergistic relationship between the factors and how their variations modulate the intensity or levels of love are discussed.

Introduction

Scholars across an array of disciplines have tried to define the meaning and nature of love with some success but questions remain. Indeed, it has been described as a propensity to think, feel, and behave positively toward another ( Hendrick and Hendrick, 1986 ). However, the application of this approach has been unsuccessful in all forms of love ( Berscheid, 2010 ). Some social psychologists have tried to define love using psychometric techniques. Robert Sternberg Triangular Theory of Love and Clyde and Susan Hendrick’s Love Attitudes Scale (LAS) are notable attempts to employ the psychometric approach ( Hendrick and Hendrick, 1986 ; Sternberg, 1986 ). However, data analysis from the administration of the LAS, Sternberg’s scale and the Passionate Love Scale by Hatfield and Sprecher’s (1986) found a poor association with all forms of love ( Hendrick and Hendrick, 1989 ). Other studies have found a poor correlation between these and other love scales with different types of love ( Whitley, 1993 ; Sternberg, 1997 ; Masuda, 2003 ; Graham and Christiansen, 2009 ).

In recent years, the neuropsychological approach to study the nature of love has gained prominence. Research has compared the brain activity of people who were deeply in love while viewing a picture of their partner and friends of the same age using functional magnetic resonance imaging (fMRI) and concluded that there is a specialized network of the brain involved in love ( Bartels and Zeki, 2000 ). Indeed, several lines of investigation using fMRI have described a specialized area of the brain mediating maternal love ( Noriuchi et al., 2008 ; Noriuchi and Kikuchi, 2013 ) and, fMRI studies have implicated multiple brain systems particularly the reward system in romantic love ( Aron et al., 2005 ; Fisher et al., 2005 , 2010 ; Beauregard et al., 2009 ). Brain regions including ventral tegmental area, anterior insula, ventral striatum, and supplementary motor area have been demonstrated to mediate social and material reward anticipation ( Gu et al., 2019 ). Although brain imaging provides a unique insight into the nature of love, making sense of the psychological significance or inference of fMRI data is problematic ( Cacioppo et al., 2003 ).

Also, there has been growing interests in the neurobiology of love. Indeed, evidence suggests possible roles for oxytocin, vasopressin, dopamine, serotonin, testosterone, cortisol, morphinergic system, and nerve growth factor in love and attachment ( Esch and Stefano, 2005 ; De Boer et al., 2012 ; Seshadri, 2016 ; Feldman, 2017 ). However, in many cases, definite proof is still lacking and the few imaging studies on love are limited by selection bias on the duration of a love affair, gender and cultural differences ( De Boer et al., 2012 ).

So, while advances have been made in unraveling the meaning of love, questions remain and a framework that can be employed to understand love in all its forms remains to be developed or proposed. The objective of this article is to propose a novel framework that can be applied to all forms of love.

Theoretical Background and Hypothesis Development (The AAC Model)

In the past few decades, the psychological literature has defined and described different forms of love and from these descriptions, the role of attraction, attachment-commitment, and caregiving (AAC), appears to be consistent in all forms of love.

Attraction theory is one of the first approaches to explain the phenomenon of love and several studies and scholarly works have described the importance of attraction in different forms of love ( Byrne and Griffitt, 1973 ; Berscheid and Hatfield, 1978 ; Fisher et al., 2006 ; Braxton-Davis, 2010 ; Grant-Jacob, 2016 ). Attraction has been described as an evolutionary adaptation of humans for mating, reproduction, and parenting ( Fisher et al., 2002a , 2006 ).

The role of attachment in love has also been extensively investigated. Attachment bonds have been described as a critical feature of mammals including parent-infant, pair-bonds, conspecifics, and peers ( Feldman, 2017 ). Indeed, neural networks including the interaction of oxytocin and dopamine in the striatum have been implicated in attachment bonds ( Feldman, 2017 ). The key features of attachment include proximity maintenance, safety and security, and separation distress ( Berscheid, 2010 ). Multiple lines of research have proposed that humans possess an innate behavioral system of attachment that is essential in love ( Harlow, 1958 ; Bowlby, 1977 , 1988 , 1989 ; Ainsworth, 1985 ; Hazan and Shaver, 1987 ; Bretherton, 1992 ; Carter, 1998 ; Burkett and Young, 2012 ). Attachment is essential to commitment and satisfaction in a relationship ( Péloquin et al., 2013 ) and commitment leads to greater intimacy ( Sternberg, 1986 ).

Also, several lines of evidence have described the role of caregiving in love. It has been proposed that humans possess an inborn caregiving system that complements their attachment system ( Bowlby, 1973 ; Ainsworth, 1985 ). Indeed, several studies have used caregiving scale and compassionate love scale, to describe the role of caring, concern, tenderness, supporting, helping, and understanding the other(s), in love and relationships ( Kunce and Shaver, 1994 ; Sprecher and Fehr, 2005 ). Mutual communally responsive relationships in which partners attend to one another’s needs and welfare with the expectation that the other will return the favor when their own needs arise ( Clark and Mills, 1979 ; Clark and Monin, 2006 ), have been described as key in all types of relationships including friendship, family, and romantic and compassionate love ( Berscheid, 2010 ).

Attachment and caregiving reinforce each other in relationships. Evidence suggests that sustained caregiving is frequently accompanied by the growth of familiarity between the caregiver and the receiver ( Bowlby, 1989 , p. 115) strengthening attachment ( Berscheid, 2010 ). Several studies have proposed that attachment has a positive influence on caregiving behavior in love and relationships ( Carnelley et al., 1996 ; Collins and Feeney, 2000 ; Feeney and Collins, 2001 ; Mikulincer, 2006 ; Canterberry and Gillath, 2012 ; Péloquin et al., 2013 ).

The AAC model can be seen across the literature on love. Robert Sternberg triangular theory of love which proposes that love has three components —intimacy, passion, and commitment ( Sternberg, 1986 ), essentially applies the AAC model. Passion, a key factor in his theory, is associated with attraction ( Berscheid and Hatfield, 1978 ), and many passionate behaviors including increased energy, focused attention, intrusive thinking, obsessive following, possessive mate guarding, goal-oriented behaviors and motivation to win and keep a preferred mating partner ( Fisher et al., 2002b , 2006 ; Fisher, 2005 ). Also, evidence indicates that attachment is central to intimacy, another pillar of the triangular theory ( Morris, 1982 ; Feeney and Noller, 1990 ; Oleson, 1996 ; Grabill and Kent, 2000 ). Commitment, the last pillar of the triangular theory, is based on interdependence and social exchange theories ( Stanley et al., 2010 ), which is connected to mutual caregiving and secure attachment.

Hendrick and Hendrick’s (1986) , Love Attitudes Scale (LAS) which measures six types of love ( Hendrick and Hendrick, 1986 ) is at its core based on the AAC model. Similarly, numerous works on love ( Rubin, 1970 ; Hatfield and Sprecher, 1986 ; Fehr, 1994 ; Grote and Frieze, 1994 ), have applied one or all of the factors in the ACC model. Berscheid (2010) , proposed four candidates for a temporal model of love including companionate love, romantic love, and compassionate love and adult attachment love. As described, these different types of love (romantic, companionate, compassionate, and attachment) all apply at least one or all of the factors in the AAC model.

New Theory (The Quadruple Framework)

The AAC model can be fully captured by four fundamental factors; attraction, connection or resonance, trust, and respect, providing a novel framework that could explain love in all its forms. Table 1 shows the core factors of love, and the four factors derived from them.

Table 1. Factors of love.

Evidence suggests that both attachment and attraction play a role in obsession or passion observed in love ( Fisher et al., 2005 ; Honari and Saremi, 2015 ). Attraction is influenced by the value or appeal perceived from a relationship and this affects commitment ( Rusbult, 1980 ).

Connection or Resonance

Connection is key to commitment, caregiving, and intimacy. It creates a sense of oneness in relationships and it is strengthened by proximity, familiarity, similarity, and positive shared experiences ( Sullivan et al., 2011 ; Beckes et al., 2013 ). Homogeneity or similarity has been observed to increase social capital and engagement among people ( Costa and Kahn, 2003a , b ), and it has been described as foundational to human relationships ( Tobore, 2018 , pp. 6–13). Research indicates that similarity plays a key role in attachment and companionship as people are more likely to form long-lasting and successful relationships with those who are more similar to themselves ( Burgess and Wallin, 1954 ; Byrne, 1971 ; Berscheid and Reis, 1998 ; Lutz-Zois et al., 2006 ). Proximity plays a key role in caregiving as people are more likely to show compassion to those they are familiar with or those closest to them ( Sprecher and Fehr, 2005 ). Similarity and proximity contribute to feelings of familiarity ( Berscheid, 2010 ). Also, caregiving and empathy are positively related to emotional interdependence ( Hatfield et al., 1994 ).

Trust is crucial for love ( Esch and Stefano, 2005 ) and it plays an important role in relationship intimacy and caregiving ( Rempel and Holmes, 1985 ; Wilson et al., 1998 ; Salazar, 2015 ), as well as attachment ( Rodriguez et al., 2015 ; Bidmon, 2017 ). Familiarity is a sine qua non for trust ( Luhmann, 1979 ), and trust is key to relationship satisfaction ( Simpson, 2007 ; Fitzpatrick and Lafontaine, 2017 ).

Respect is cross-cultural and universal ( Frei and Shaver, 2002 ; Hendrick et al., 2010 ) and has been described as fundamental in love ( Hendrick et al., 2011 ). It plays a cardinal role in interpersonal relations at all levels ( Hendrick et al., 2010 ). Indeed, it is essential in relationship commitment and satisfaction ( Hendrick and Hendrick, 2006 ) and relationship intimacy and attachment ( Alper, 2004 ; Hendrick et al., 2011 ).

Synergetic Interactions of the Four Factors

Connection and attraction.

Similarity, proximity, and familiarity are all important in connection because they promote attachment and a sense of oneness in a relationship ( Sullivan et al., 2011 ; Beckes et al., 2013 ). Research indicates that proximity ( Batool and Malik, 2010 ) and familiarity positively influence attraction ( Norton et al., 2015 ) and several lines of evidence suggests that people are attracted to those similar to themselves ( Sykes et al., 1976 ; Wetzel and Insko, 1982 ; Montoya et al., 2008 ; Batool and Malik, 2010 ; Collisson and Howell, 2014 ). Also, attraction mediates similarity and familiarity ( Moreland and Zajonc, 1982 ; Elbedweihy et al., 2016 ).

Respect and Trust

Evidence suggests that respect promotes trust ( Ali et al., 2012 ).

Connection, Respect, Trust, and Attraction

Trust affects attraction ( Singh et al., 2015 ). Trust and respect can mediate attitude similarity and promote attraction ( Singh et al., 2016 ).

So, although these factors can operate independently, evidence suggests that the weakening of one factor could negatively affect the others and the status of love. Similarly, the strengthening of one factor positively modulates the others and the status of love.

Relationships are dynamic and change as events and conditions in the environment change ( Berscheid, 2010 ). Love is associated with causal conditions that respond to these changes favorably or negatively ( Berscheid, 2010 ). In other words, as conditions change, and these factors become present, love is achieved and if they die, it fades. Figure 1 below explains how love grows and dies. Point C in the figure explains the variations in the intensity or levels of love and this variation is influenced by the strength of each factor. The stronger the presence of all factors, the higher the intensity and the lower, the weaker the intensity of love. The concept of non-love is similar to the “non-love” described in Sternberg’s triangular theory of love in which all components of love are absent ( Sternberg, 1986 ).

Figure 1. Description: (A) Presence of love (all factors are present). (B) Absence of love (state of non-love or state where all factors are latent or dormant). (C) Different levels of love due to variations in the four factors. (D) Movement from non-love toward love (developmental stage: at least one but not all four factors are present). (E) Movement away from love toward non-love (decline stage: at least one or more of the four factors are absent).

Application of the Quadruple Framework on Romantic, Brand and Parental Love

Romantic, parental and brand love have been chosen to demonstrate the role of these factors and their interactions in love because there is significant existing literature on them. However, they can be applied to understand love in all its forms.

Romantic Love

Attraction and romantic love.

Attraction involves both physical and personality traits ( Braxton-Davis, 2010 ; Karandashev and Fata, 2014 ). To this end, attraction could be subdivided into sexual or material and non-sexual or non-material attraction. Sexual or material attraction includes physical attributes such as beauty, aesthetics, appeal, wealth, etc. In contrast, non-sexual or non-material attraction includes characteristics such as personality, social status, power, humor, intelligence, character, confidence, temperament, honesty, good quality, kindness, integrity, etc. Both types of attraction are not mutually exclusive.

Romantic love has been described as a advanced form of human attraction system ( Fisher et al., 2005 ) and it fits with the passion component of Sternberg’s triangular theory of love which he described as the quickest to recruit ( Sternberg, 1986 ). Indeed, research indicates that physical attractiveness and sensual feelings are essential in romantic love and dating ( Brislin and Lewis, 1968 ; Regan and Berscheid, 1999 ; Luo and Zhang, 2009 ; Braxton-Davis, 2010 ; Ha et al., 2010 ; Guéguen and Lamy, 2012 ) and sexual attraction often provides the motivational spark that kickstarts a romantic relationship ( Gillath et al., 2008 ). Behavioral data suggest that love and sex drive follow complementary pathways in the brain ( Seshadri, 2016 ). Indeed, the neuroendocrine system for sexual attraction and attachment appears to work synergistically motivating individuals to both prefer a specific mating partner and to form an attachment to that partner ( Seshadri, 2016 ). Sex promotes the activity of hormones involved in love including arginine vasopressin in the ventral pallidum, oxytocin in the nucleus accumbens and stimulates dopamine release which consequently motivates preference for a partner and strengthens attachment or pair-bonding ( Seshadri, 2016 ).

Also, romantic love is associated with non-material attraction. Research indicates that many people are attracted to their romantic partner because of personality traits like generosity, kindness, warmth, humor, helpfulness, openness to new ideas ( Giles, 2015 , pp. 168–169). Findings from a research study on preferences in human mate selection indicate that personality traits such as kindness/considerate and understanding, exciting, and intelligent are strongly preferred in a potential mate ( Buss and Barnes, 1986 ). Indeed, character and physical attractiveness have been found to contribute jointly and significantly to romantic attraction ( McKelvie and Matthews, 1976 ).

Attraction is key to commitment in a romantic relationship ( Rusbult, 1980 ), indicating that without attraction a romantic relationship could lose its luster. Also, romantic attraction is weakened or declines as the reason for its presence declines or deteriorates. If attraction is sexual or due to material characteristics, then aging or any accident that compromises physical beauty would result in its decline ( Braxton-Davis, 2010 ). Loss of fortune or social status could also weaken attraction and increase tension in a relationship. Indeed, tensions about money increase marital conflicts ( Papp et al., 2009 ; Dew and Dakin, 2011 ) and predicted subsequent divorce ( Amato and Rogers, 1997 ).

Connection and Romantic Love

Connection or resonance fits with the intimacy, and commitment components of Sternberg’s triangular theory of love ( Sternberg, 1986 ). Connection in romantic love involves intimacy, friendship or companionship and caregiving and it is strengthened by novelty, proximity, communication, positive shared experiences, familiarity, and similarity. It is what creates a sense of oneness between romantic partners and it is expressed in the form of proximity seeking and maintenance, concern, and compassion ( Neto, 2012 ). Evidence suggests that deeper levels of emotional involvement or attachment increase commitment and cognitive interdependence or tendency to think about the relationship in a pluralistic manner, as reflected in the use of plural pronouns to describe oneself, romantic partner and relationship ( Agnew et al., 1998 ).

Research indicates that both sexual attraction and friendship are necessary for romantic love ( Meyers and Berscheid, 1997 ; Gillath et al., 2008 ; Berscheid, 2010 ), indicating that connection which is essential for companionship plays a key role in romantic love. A study on college students by Hendrick and Hendrick (1993) found that a significant number of the students described their romantic partner as their closest friend ( Hendrick and Hendrick, 1993 ), reinforcing the importance of friendship or companionship in romantic love.

Similarity along the lines of values, goals, religion, nationality, career, culture, socioeconomic status, ethnicity, language, etc. is essential in liking and friendship in romantic love ( Berscheid and Reis, 1998 ). Research indicates that a partner who shared similar values and interests were more likely to experience stronger love ( Jin et al., 2017 ). Indeed, the more satisfied individuals were with their friendships the more similar they perceived their friends to be to themselves ( Morry, 2005 ). Also, similarity influences perceptions of familiarity ( Moreland and Zajonc, 1982 ), and familiarity plays a role in the formation of attachment and connectedness because it signals safety and security ( Bowlby, 1977 ). Moreover, similarity and familiarity affect caregiving. Sprecher and Fehr (2005) , found compassion or caregiving were lower for strangers, and greatest for dating and marital relationships, indicating that similarity and familiarity enhance intimacy and positively influences caregiving ( Sprecher and Fehr, 2005 ).

Proximity through increased exposure is known to promote liking ( Saegert et al., 1973 ), familiarity and emotional connectedness ( Sternberg, 1986 ; Berscheid, 2010 ). Exposure through fun times and direct and frequent communication is essential to maintaining and strengthening attachment and connectedness ( Sternberg and Grajek, 1984 ). In Sternberg’s triangular theory, effective communication is described as essential and affects the intimacy component of a relationship ( Sternberg, 1986 ). Indeed, intimacy grows from a combination of mutual self-disclosure and interactions mediated by positive partner responsiveness ( Laurenceau et al., 1998 , 2005 ; Manne et al., 2004 ), indicating that positive feedback and fun times together strengthens connection.

Also, sexual activity is an important component of the reward system that reinforces emotional attachment ( Seshadri, 2016 ), indicating that sexual activity may increase emotional connectedness and intimacy. Over time in most relationships, predictability grows, and sexual satisfaction becomes readily available. This weakens the erotic and emotional experience associated with romantic love ( Berscheid, 2010 ). Research shows that a reduction in novelty due to the monotony of being with the same person for a long period is the reason for this decline in sexual attraction ( Freud and Rieff, 1997 , p. 57; Sprecher et al., 2006 , p. 467). According to Sternberg (1986) , the worst enemy of the intimacy component of love is stagnation. He explained that too much predictability can erode the level of intimacy in a close relationship ( Sternberg, 1986 ). So, novelty is essential to maintaining sexual attraction and strengthening connection in romantic love.

Jealousy and separation distress which are key features of romantic love ( Fisher et al., 2002b ), are actions to maintain and protect the emotional union and are expressions of a strong connection. Research has found a significant correlation between anxiety and love ( Hatfield et al., 1989 ) and a positive link between romantic love and jealousy in stable relationships ( Mathes and Severa, 1981 ; Aune and Comstock, 1991 ; Attridge, 2013 ; Gomillion et al., 2014 ). Indeed, individuals who feel strong romantic love tend to be more jealous or sensitive to threats to their relationship ( Orosz et al., 2015 ).

Connection in romantic love is weakened by distance, a dearth of communication, unsatisfactory sexual activity, divergences or dissimilarity of values and interests, monotony and too much predictability.

Trust and Romantic Love

Trust is the belief that a partner is, and will remain, reliable or dependable ( Cook, 2003 ). Trust in romantic love fits with the intimacy, and commitment components of Sternberg’s triangular theory of love which includes being able to count on the loved one in times of need, mutual understanding with the loved one, sharing of one’s self and one’s possessions with the loved one and maintaining the relationship ( Sternberg, 1986 ).

It has been proposed that love activates specific regions in the reward system which results in a reduction in emotional judgment and fear ( Seshadri, 2016 ). This reduced fear or trust has been identified as one of the most important characteristics of a romantic relationship and essential to fidelity, commitment, monogamy, emotional vulnerability, and intimacy ( Laborde et al., 2014 ). Indeed, trust can deepen intimacy, increase commitment and increase mutual monogamy, and make a person lower their guards in the belief that they are safe from harm ( Larzelere and Huston, 1980 ; Bauman and Berman, 2005 ). People with high trust in romantic relationships tend to expect that their partner will act in their interest causing them to prioritize relationship dependence over making themselves invulnerable from harm or self-protection ( Luchies et al., 2013 ). In contrast, people with low trust in their partner tend to be unsure about whether their partner will act in their interests and prioritize insulating themselves from harm over relationship dependence ( Luchies et al., 2013 ).

Trust takes time to grow into a romantic relationship. Indeed, people in a relationship come to trust their partners when they see that their partner’s action and behavior moves the relationship forward or acts in the interest of the relationship and not themself ( Wieselquist et al., 1999 ). Research indicates that trust is associated with mutual self-disclosure ( Larzelere and Huston, 1980 ), and positive partner responsiveness which are both essential to the experience of friendship and intimacy in romantic relationships ( Larzelere and Huston, 1980 ; Reis and Shaver, 1988 ; Laurenceau et al., 1998 ).

Also, trust influences caregiving and compassion. Evidence suggests that compassion is positively related to trust ( Salazar, 2015 ). Mutual communal responsiveness or caregiving in relationships in which partners attend to one another’s needs and welfare is done because they are confident that the other will do the same when or if their own needs arise ( Clark and Monin, 2006 ). Repeated acts of communal responsiveness given with no expectation of payback provide a partner with a sense of security and trust and increase the likelihood that they will be communally responsive if or when the need arises ( Clark and Monin, 2006 ), and contributes to a sense of love in romantic relationships ( Berscheid, 2010 ).

Loss or weakening of trust could spell the end of romantic love. Indeed, mistrust corrupts intimacy and often indicates that a relationship has ended or near its end ( LaFollette and Graham, 1986 ) and it makes mutual monogamy, and commitment difficult to achieve in a romantic relationship ( Towner et al., 2015 ). A study on individuals who had fallen out of romantic love with their spouse found that loss of trust and intimacy was part of the reason for the dissolution of love ( Sailor, 2013 ).

Respect and Romantic Love

Multiple lines of evidence suggest that respect is expected in both friendships and romantic relationships ( Gaines, 1994 , 1996 ). In romantic love, it entails consideration, admiration, high regard, and value for the loved one as a part of one’s life ( Sternberg and Grajek, 1984 ; Hendrick et al., 2011 ).

Gottman (1999) , found that the basis for a stable and satisfactory marital relationship is friendship filled with fondness and admiration ( Gottman, 1999 ). Respect is considered one of the most important things married couples want from their partner ( Gottman, 1994 ). Grote and Frieze (1994) , found that respect correlates with companionate or friendship love ( Grote and Frieze, 1994 ), indicating that respect is essential to intimacy and relationship satisfaction. Also, respect is positively correlated with passion, altruism, self-disclosure, and relationship overall satisfaction ( Frei and Shaver, 2002 ; Hendrick and Hendrick, 2006 ). It is associated with the tendency to overlook a partner’s negative behavior or respond with pro-relationship actions or compassion to their shortcomings ( Rusbult et al., 1998 ; Gottman, 1999 ).

Absence or a lack of respect could spell the end of romantic love. Research indicates that there is an expectation of mutual respect in friendship and most relationships and people reacted negatively when this expectation is violated ( Hendrick et al., 2011 ), indicating that a lack of respect could negatively affect commitment and attraction. Indeed, denial of respect is an important negative behavior in friendships and most relationships ( Gaines, 1994 , 1996 ) and a lack of respect is a violation of what it means to love one ‘s partner in a close romantic relationship ( Hendrick et al., 2011 ). Gottman (1993 , 1994) identified contempt, criticism, defensiveness, and stonewalling as four of the relationally destructive behavior and he labeled them as “the four horsemen of the apocalypse.”

Romantic love summary

Romantic love involves the interactions and synergistic interplay between respect, connection, trust, and attraction. All four must be present in love. Any event that results in the loss of any of these factors could cause romantic love to gradually decline and unless effort is made to replenish it, it will eventually fade or collapse. Romantic love is dynamic and requires significant investment from both partners to keep it alive.

Parental Love

Attraction and parental love.

Attraction plays an essential role in parental love and it could be material or non-material. Material attraction involves the child’s health, gender, accomplishments or success, and attractiveness. In contrast, non-material attraction includes traits such as intelligence, character, and other personality traits.

Evidence suggests that culture influences gender preference with attraction greater for sons in most cases ( Cronk, 1993 ). Indeed, mothers and fathers have been found to favor the more intelligent and more ambitious/industrious child ( Lauricella, 2009 ). Also, parental perception that investment in a child will cost more than the benefits to be gained from taking care of the child might influence negative behavior toward the child. Indeed, multiple lines of evidence suggest that parental unemployment increases the rates of child maltreatment and abuse ( Steinberg et al., 1981 ; Lindo et al., 2013 ). Research indicates that teen mothers who have poor social support reported greater unhappiness, were at greater risk for child abuse and often employed the use of physical punishment toward their child ( Haskett et al., 1994 ; de Paúl and Domenech, 2000 ).

Also, several studies have suggested that parents tended to favor healthy children ( Mann, 1992 ; Barratt et al., 1996 ; Hagen, 1999 ). However, when resources are plentiful, parents tend to invest equally in less healthy or high-risk children ( Beaulieu and Bugental, 2008 ), because they have abundant resources to go around without compromising the reproductive value of healthy children ( Lauricella, 2009 ).

Connection and Parental Love

Connection creates a sense of oneness between parent and child and involves caregiving, intimacy, and attachment. It is influenced by proximity, positive and unique shared experiences, and similarity along virtually every dimension between parent and child.

Proximity, and similarity increases attachment and intimacy between parent and child. Research shows that parents are perceived as favoring genetically related children ( Salmon et al., 2012 ), and evidence suggests that paternal resemblance predicted paternal favoritism ( Lauricella, 2009 ). Parental proximity and similarity to a biological child are unique because it is based on genes and blood. In contrast, intimacy between a parent and an adopted child is based solely on shared experiences and proximity and takes time to grow and on many occasions may not develop ( Hooks, 1990 ; Hughes, 1999 ).

Dissimilarities or discrepancy in values, attitudes, etc., can create problems between children and parents and can have a profound effect on their relationship. Indeed, evidence suggests that the rebel child tended to be less close to the parents ( Rohde et al., 2003 ). Research has found that adolescents who are less religious than their parents tend to experience lower-quality relationships with their parents which results in higher rates of both internalizing and externalizing symptoms ( Kim-Spoon et al., 2012 ). When parents and family members were very religious, and a child comes out as an atheist, relationship quality could suffer in the form of rejection, anger, despair, or an inability to relate to one another ( Zimmerman et al., 2015 ). A study of lesbian, gay, and bisexual youngsters, for patterns of disclosure of sexual orientation to families, found that those who had disclosed reported verbal and physical abuse by parents and family members ( D’Augelli et al., 1998 ). Honor killing of female children which have been reported in Pakistan and some parts of the Middle East because of deviation from traditional gender roles or crossing of social boundaries that are deemed as taboo in their culture ( Lindsey and Sarah, 2010 ), is another example of the negative effects of the discrepancy in values between parents and child.

Unique shared experiences between parent and child could increase connection. Bank (1988) observed that the development of favoritism seems to require that the “child’s conception or birth be unusual or stressful,” ( Bank, 1988 ). Evidence suggests that parents most favored child tended to be last-born child and this is linked to their unique position, vulnerability and neediness ( Rohde et al., 2003 ). Also, proximity, positive experiences and time spent together increases connection and intimacy. Research indicates that parents tend to give more love and support to the grown child they were historically closest to and got along with ( Siennick, 2013 ). A study of primiparous women found that mothers with greater contact with their infants were more reluctant to leave them with someone else, and engaged more intimately with their child ( Klaus et al., 1972 ).

Divorce could create distance between a parent and child, weakening connection and intimacy. Indeed, one of the outcomes of divorce is the lessening of contact between divorced non-custodial fathers and their children ( Appleby and Palkovitz, 2007 ), and this can reduce intimacy ( Guttmann and Rosenberg, 2003 ).

Also, parental separation distress, worry, and concern for their child’s welfare, academic performance, and future are expressions of connection and a lack thereof is a sign of poor connection. Indeed, the levels of concern and worry expressed between children and their parents influenced their perceptions of the relationship quality ( Hay et al., 2007 ).

Trust and Parental Love

Trust is essential to parental attachment, intimacy, and caregiving. When there is mistrust, attachment and intimacy between a parent and their child are disrupted or unable to blossom. In Africa and many parts of the world, there have been reports of children being condemned and abandoned by their parents simply because they are tagged as witches with mysterious evil powers ( Tedam, 2014 ; Bartholomew, 2015 ; Briggs and Whittaker, 2018 ). The tag of “witchcraft” stirs up fear and anger, causing the child to be perceived as a deadly threat which inevitably damages attachment, intimacy and eliminates the need for caregiving.

Research has found that firstborn children were most likely to be chosen as those to whom mothers would turn when facing personal problems or crises ( Suitor and Pillemer, 2007 ). This tendency may be linked to trust. Moreover, evidence suggests that the rebel child tended to be less close to the parents ( Rohde et al., 2003 ). In other words, the more obedient, and reliable child is likely to gain the confidence and intimacy of the parents. In contrast, the disobedient and unreliable child is excluded or kept at a distance. Also, trust and poor connection could influence inheritance and disinheritance decisions. Indeed, estrangement, alienation and disaffection of a parent toward a child could result in disinheritance ( Batts, 1990 ; Brashier, 1994 , 1996 ; Foster, 2001 ; Arroyo et al., 2016 ).

Respect and Parental Love

Respect in parental love entails treating the child with consideration and regard. This consideration and regard for the child are essential to intimacy, caregiving and attachment. Indeed, respect is foundational to a harmonious relationship between parent and child ( Dixon et al., 2008 ). Evidence suggests that humans possess an innate behavioral system that leads them to form an attachment to a familiar person who provides care, comfort, and protection ( Harlow, 1958 ; Bowlby, 1989 ). Repeated acts of caregiving contribute to a sense of love in all types of relationships ( Berscheid, 2010 ), reinforcing the role of parental caregiving in fostering intimacy and attachment with the child.

Taking care of an infant’s needs, and making sure they are safe and well, all fall under consideration and regard for the child. Child abuse and neglect ( Tedam, 2014 ; Bartholomew, 2015 ; Briggs and Whittaker, 2018 ), is a display of a lack of consideration for the child’s need.

Also, respect in parental love involves admiration. Research has found that fathers treated more ambitious/industrious sons with high regard, and both parents favored the more intelligent and more ambitious/industrious daughters ( Lauricella, 2009 ) indicating that a child that engages in activities or behavior that is highly regarded by their parents may gain favor with their parents, strengthening intimacy and vice versa.

Parental love summary

Parental love involves the interactions and synergistic interplay between respect, connection, trust, and attraction. Any event that results in the loss of any of these factors could cause parental love to gradually decline. In many cases, the behavior and actions of a child significantly influence parental love.

Brand love has been defined as the level of passionate emotional attachment a satisfied or happy consumer has for a brand and evidence suggests it is very similar to interpersonal love ( Russo et al., 2011 ).

Attraction and Brand Love

Attraction plays an essential role in brand love. Material attraction for a brand includes attributes like superior design, quality, and aesthetics, price, benefits, etc. Non-material attraction involves social status symbol, brand personality, uniqueness, distinctiveness, user experience, image, etc. evidence suggests that when talking about loved brands, people often talk passionately about the brand’s many attractive qualities such as its exceptional performance, good-looking design, value for money, and other positive attributes ( Fournier, 1998 ; Whang et al., 2004 ; Carroll and Ahuvia, 2006 ; Batra et al., 2012 ). Research on brand love has found that brand attractive attributes such as prestige or uniqueness influence brand passion which affects relevant factors such as purchase intention ( Bauer et al., 2007 ).

Also, brand attraction influences brand loyalty, and commitment. Indeed, research indicates that brand benefits influences brand loyalty or commitment ( Huang et al., 2016 ). Brand personality (image, distinctiveness, and self-expressive value) is strongly associated with brand identification and loyalty ( Kim et al., 2001 ; Elbedweihy et al., 2016 ).

Connection and Brand Love

Connection is essential to brand love. It involves brand attachment, commitment, and intimacy and it is strengthened by brand identification, image, familiarity or awareness, proximity, length or frequency of usage and similarity or congruences along virtually every dimension including values, lifestyle, goals, etc. between brand and customer. Brand awareness which means brand familiarity has been described as essential for people to identify with a brand ( Pascual and Académico, 2015 ), and it indirectly affects current purchases ( Esch et al., 2006 ).

Also, brand identification promotes a sense of oneness between a brand and a customer strengthening commitment and it is driven by brand self-similarity, brand prestige and brand distinctiveness ( Stokburger-Sauer et al., 2008 ). Indeed, brand identification contributes to the development of brand love and brand loyalty ( Alnawas and Altarifi, 2016 ) and brand image and identification influence loyalty and positive word of mouth ( Carroll and Ahuvia, 2006 ; Batra et al., 2012 ; Anggraeni and Rachmanita, 2015 ). Brand identity, values and lifestyle similarities to those of the customer appear to have a strong and significant relationship with brand love ( Batra et al., 2012 ; Rauschnabel and Ahuvia, 2014 ; Alnawas and Altarifi, 2016 ; Elbedweihy et al., 2016 ). Findings from research suggest that customer-to-customer similarity and sense of community drive consumer brand identification, loyalty, and engagement ( Bergkvist and Bech-Larsen, 2010 ; Elbedweihy et al., 2016 ).

Moreover, proximity and interaction play a role in brand love. Indeed, the duration of the relationship between a customer and a brand is essential in brand love ( Albert et al., 2007 ). Fournier (1998) , discussed interdependence which involved frequent brand interactions as necessary for a strong brand relationship ( Fournier, 1998 ). Similarly, Batra et al. (2012) found that having a long-term relationship, positive emotional connection and frequent interactions with a brand was an important aspect of brand love ( Batra et al., 2012 ). Indeed, shared experiences and history between a person and a brand can increase their emotional attachment, make the brand to become an important part of the person’s identity narrative and increases their loyalty to the brand ( Thomson et al., 2005 ; Pedeliento et al., 2016 ).

Just like romantic love, concern and worry and proximity seeking, or maintenance are an expression of emotional connectedness to the brand. Indeed, anticipated separation distress has been described as a core element of brand love ( Batra et al., 2012 ), and consumers are likely to feel strong desires to maintain proximity with their loved objects, even feeling “separation distress” when they are distanced from them ( Thomson et al., 2005 ; Park et al., 2010 ).

Also, novelty through continued innovation is vital to maintaining and strengthening both attraction and connection. According to the Harvard business review, the relationship between brand and consumer go through “ruts” and to “keep the spark” alive, innovation and news are essential ( Halloran, 2014 ). Research indicates that innovation plays a role in brand equity and it impacts brand identification or resonance ( Sinha, 2017 ).

Lack of brand familiarity or awareness, poor or negative user experience, a dearth of innovation and increased dissimilarities in values and lifestyles between brand and consumer can all weaken brand connection.

Trust and Brand Love

Trust is essential to brand attachment, intimacy, and commitment. It involves confidence and reliability, or dependability of the brand and it is influenced by brand image, familiarity, values, user experience, and quality. Indeed, brand trust directly influences brand love ( Turgut and Gultekin, 2015 ; Meisenzahl, 2017 ) and a strong relationship exists between brand love and brand trust and identification ( Albert and Merunka, 2013 ). Evidence suggests that brand familiarity influences brand trust ( Ha and Perks, 2005 ) and brand trust and experience, positively influence brand attachment ( Erciş et al., 2012 ; Chinomona, 2013 ; Chinomona and Maziriri, 2017 ).

Also, brand trust affects brand purchase, loyalty, and commitment. Evidence suggests that a strong relationship exists between brand love and brand trust, brand commitment, positive word of mouth, and willingness to pay a higher price for the brand ( Albert and Merunka, 2013 ). Research indicates that brand trust positively affects brand loyalty ( Setyawan and Kussudiyarsana, 2015 ), directly influences brand purchase intentions ( Yasin and Shamim, 2013 ) and positively influences current and future purchases ( Erciş et al., 2012 ). Indeed, more than any other factor, brand trust has been identified as essential for future purchases of a brand ( Esch et al., 2006 ). It is essential in determining purchase loyalty and attitudinal loyalty and it plays a role in brand market share ( Chaudhuri and Holbrook, 2001 ). Brand trust affects both affective and continuance commitment and affective commitment influences repurchase intention and loyalty ( Erciş et al., 2012 ).

Brand quality is essential to brand trust and love. Indeed, Fournier (1998) , discussed the role of brand quality in brand love and highlighted the role of trust in relationship satisfaction and strength ( Fournier, 1998 ). Also, brand trust has been found to positively affect resistance to negative information and repurchase intention ( Turgut and Gultekin, 2015 ).

Brand trust is weakened by poor user experience, brand quality, brand image, and a lack of brand familiarity.

Respect and Brand Love

Brand respect is essential in brand love and plays an important role in brand attachment, intimacy, and commitment. It is influenced by brand identification, values, image, experience, and quality. Brand respect is displayed by the customer in the form of high regard, admiration for the brand, brand loyalty and consideration or tolerance of negative information. Indeed, brand familiarity positively affects brand respect ( Zhou, 2017 ), indicating that brand familiarity increases regard for a brand. Evidence suggests that brand image positively influences brand respect and love ( Cho, 2011 ), indicating that brand image modulates a customer’s regard and admiration for a brand.

Brand respect influences brand commitment and loyalty. Indeed, a strong relationship has been found between brand respect and brand loyalty ( Cho, 2011 ) and brand admiration results in greater brand loyalty, stronger brand advocacy, and higher brand equity ( Park et al., 2016 ). Brand respect affects the behavioral outcomes of brand love such as affective commitment, and willingness to pay a price premium ( Garg et al., 2016 ; Park et al., 2016 ).

Also, evidence suggests that customers’ admiration or high regard for a brand contributes to why they tend to ignore negative information about the brand ( Elbedweihy et al., 2016 ). Fournier (1998) , included respect as one of the components of brand partner quality. This means that respect is one of the factors that reflects the consumer’s evaluation of the brand’s performance ( Fournier, 1998 ).

A lack of respect could negatively influence the relationship between a brand and a customer. Indeed, people react negatively when the expectation of respect is violated ( Hendrick et al., 2011 ) and a violation of expectation between brand and customer has been found to contribute to brand hate ( Zarantonello et al., 2016 ).

Brand love summary

Brand love involves the interactions and synergistic interplay between respect, connection, trust, and attraction. Any event that results in the loss of any of these factors could cause brand love to gradually decline and unless effort is made to replenish it, it will eventually fade or collapse. Brand love is dynamic and requires significant investment from the brand to keep it alive.

Strengths and Advances Made by the Quadruple Theory

The quadruple theory builds on many of the strengths of previous theories of love and it applies a temporal approach that has been proposed as the best way to understand love ( Berscheid, 2010 ). It goes further than previous theories for several reasons. Firstly, it could potentially be applied to any form of love although, only brand, romantic and parental love were discussed in this paper due to the paucity of scholarly articles on other forms of love. One of the reasons current love scales and approaches have been unable to be applied in all forms of love ( Hendrick and Hendrick, 1989 ; Whitley, 1993 ; Sternberg, 1997 ; Masuda, 2003 ; Graham and Christiansen, 2009 ), is because they capture only a part of the ACC model, unlike the quadruple framework which fully captures it.

Unlike previous theories, the quadruple theory’s application of the complex factor of connection/resonance gives it an edge in furthering our understanding of love. Proximity, positive shared experience, familiarity, and similarity are vital to connection and connection has the most profound influence on all the other factors.

Also, the dynamism and variation of these factors provide a fresh way to understand love from its development to collapse. As Figure 1 shows, love tends to take time to mature in a relationship and can die as these factors rise and decline. Figure 1 shows that variations in the presence of these factors represent different levels of love. Love in any relationship is influenced by the events in the environment it is embedded, and it responds favorably or negatively to these changes. Indeed, people get sick, old, lose their finances, travel in search of greener pastures creating distance, develop new interests different from their partner’s and all these influences the presence and absence of love. One brand becomes more innovative, improves its product quality and users experience over another and people gradually love it more than the one they previously loved. In other words, love is very dynamic and may be divided into high, moderate and low. Another point highlighted in Figure 1 is that the absence of one factor represents the absence of love and only the presence of all factors represents the presence of love. Indeed, the decline of a factor can be replenished in response to changes in the environment causing the reestablishment of love. Trust could decline but attraction and respect remain and over time trust could be replenished.

This dynamic understanding of love implies that it can be nurtured and sustained. As an example, for a brand to be loved and to maintain that love, it must make products that are attractive (appealing). It must be able to connect to its target customers by reaching out through adverts to achieve familiarity and it must ensure that its values, goals, actions are consistently similar to those of its customer base. Also, it must ensure its services and products and actions promote and maintain trust with its customers. It must respect (value) its customer’s interests and ensure that its services and products continue to receive the admiration of its customers. Table 2 describes how brand love can be nurtured and preserved.

Table 2. Brand love can be nurtured and maintained.

Using this framework, a love scale or algorithm could be developed to ascertain the presence or absence of love in any relationship. Such a scale must effectively capture these four factors and must consider the type of love being calculated in its approach. As an example, in trying to create a scale for romantic love, sexual attraction, and activity may be important for attraction and connection (depending on the age of the partners) but would be unnecessary in the calculation of brand or parental love.

Major Challenges for the Theory

One of the biggest challenges the theory faces is the lack of psychometric data to prove many of its claims. Most of its arguments are based on decades of psychological data, but its lack of psychometric data weakens the theory significantly. Also, the entire premise of the theory is based on the ACC model, which has not been validated as essential or foundational to understanding love. Perhaps, something else needs to be added to the model that the theory may have missed. The argument that the quadruple theory captures the ACC model better than previous theories on love is an argument that has not been validated, and it remains to be seen if this is true. Also, the argument that it can be applied to all forms of love apart from the three discussed remains to be tested and verified.

Gaps currently exist in our understanding of love and evidences from the existing literature show that a framework that can be applied to all forms of love is needed. The quadruple theory hopes to be that framework. It is likely to broaden our understanding of the complex nature of love. It could make love less complex by making it something that can be cultivated or nurtured, regulated and preserved. Future research should consider the modulatory roles of peptides, neurotransmitters, and hormones on these factors and their influence on love as well as the integrated parts of the brain that modulates all these factors and how they work synergistically in different stages of love.

It is important to note that love is universal and applies to people of all cultures, races, ethnicities, religion and sexual orientations. Indeed, romantic love as described by the quadruple theory applies equally to heterosexual relationships and to the relationships of people in the LGTBQ community.

In conclusion, culture has a monumental influence on what people feel, think, and how they behave toward other people and things in their environment ( Karandashev, 2015 ; Ching Hei and David, 2018 ). So, it can be considered a modulating factor on the factors discussed and on love.

Author Contributions

The author confirms being the sole contributor of this work and has approved it for publication.

Conflict of Interest

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Keywords : triangular theory of love, Romance, brand love, parental love, maternal love, Meaning of love, Definition of love, am I in love

Citation: Tobore TO (2020) Towards a Comprehensive Theory of Love: The Quadruple Theory. Front. Psychol. 11:862. doi: 10.3389/fpsyg.2020.00862

Received: 20 September 2019; Accepted: 07 April 2020; Published: 19 May 2020.

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*Correspondence: Tobore Onojighofia Tobore, [email protected]

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Science of love, theories of love in the ancient world.

It is easy to think of love in purely romantic terms, but the theme of love occurs in a variety of contexts in Greek and Roman thought. From cosmological speculation on the structure of the universe to medical theories of the emotions, from philosophical investigations into the nature of friendship to poetic accounts of the birth of the gods, love has a central role to play.

Diogenes Laertius (c. 3 rd century CE)

Les vies des plus illustres philosophes de l'antiquité .

Aamsterdam: Chez J. H. Schneider, 1758.

PA3965.D6 .A2 1758 2

Empedocles of Acragas, modern Agrigento, was a prominent pre-Socratic philosopher who proposed that the world is made up of four elements—air, earth, fire, and water—that combine and separate under the influence of two divine forces: strife and love. His writings survive only in fragments, including several passages quoted by Diogenes Laertius. Diogenes, who wrote biographies of many ancient philosophers, also reports that Empedocles died by leaping into a volcano to prove that he had become immortal.

Plato (428/427 – 348/347 BCE)

Omnia divini Platonis opera .

Basileae: In Officina Frobeniana, 1532.

PA4280 A4 1532

The Greek philosopher Plato wrote a number works of the topic of love. The most famous of these is the Symposium , in which a series of distinguished Athenians attending a banquet deliver speeches on the theme of love. Particularly well known is the speech of the comic playwright Aristophanes, who proposes that humans once had bodies made up of what would now be two humans, with four arms and legs and two faces looking in opposite directions (right). Zeus split these original humans in two as a punishment for their hubris, and Aristophanes argues that human love resulted from this splitting and reflects the desire to find one’s ‘other half’. Almost unknown in medieval Europe, Plato’s works were reintroduced into Western Europe in the original Greek in the 15 th century, but it was Marsilio Ficino’s translation into Latin (left) that made Plato accessible to a wide audience.

Lysis, or Friendship; Symposium; Phaedrus .

[Mount Vernon]: A. Colish, 1968.

B358 .J865 1968

Titus Lucretius Carus (99 – 55 BCE)

De rerum natura .

Los Angeles: Ward Ritchie Press, 1957.

Rare PA6483 .E5 L4 1957

According to St. Jerome, Lucretius went mad after drinking a love potion and wrote De rerum natura during brief spells of sanity. It is unlikely that there is much truth in Jerome’s account, but love plays a central role in Lucretius’ poem nevertheless. De rerum natura begins with an invocation of Venus, goddess of love, who is viewed as the prime generative force in nature, but elsewhere in the poem he describes love as a festering wound in need of a cure.

Lucius Annaeus Seneca (c. 4 BCE – 65 CE)

Opera qvae exstant omnia .

Antverpiae: Ex Officina Platiniana B. Moreti, 1652.

Rare Folio PA6661 .A2 1652

The Stoic philosopher Seneca discusses many kinds of love in his works – love of one’s family, love of one’s country, love of one’s self – but it is the relationship between love and friendship that particularly interests him. Love, he says, is “friendship gone mad”. This edition of Seneca’s complete works was edited by the famous Flemish humanist Justus Lipsius, who receives a magnificent portrait on the frontispiece. Seneca himself makes do with a much smaller portrait at the bottom of the title page.

Plotinus (204/205 – 270 CE)

Plotini opera omnia .

Oxonii: E Typographeo Academico, 1835.

PA4363 .A2 1835

Like earlier thinkers such as Empedocles and Lucretius, the Neoplatonist philosopher Plotinus took a cosmic view of love. Although Plotinus closely associates love with beauty, Plotinus makes love as much a part of his metaphysics and epistemology as of his aesthetic theory. True love is the endpoint of a journey undertaken through the perception and contemplation of Beauty. Beauty is itself a property of the Forms, eternal and unchanging entities that underlie everything that is intelligible in the universe. This scholarly edition of Plotinus, beautifully printed at Oxford University, includes both the original Greek text and a Latin translation.

Pseudo-Galen (c. 3 rd century CE)

Microtegni .

France: 13 th century.

The emotions, including love, did not play a large role in ancient medicine. They were almost always explained reductively as the result of an imbalance in the four humors – black bile, yellow bile, phlegm, and blood – that formed the basis of ancient medical theory. Occasionally, however, emotions were recognized as a cause of illness. In a famous case, the doctor and medical writer Galen concluded that a women suffering from a mysterious ailment had, in fact, simply fallen in love. These two manuscript fragments are both indirectly associated with Galen. The Microtegni may well be identical with a treatise called the De spermate , written in late antiquity and at some point attributed to Galen himself. The Treatise on Tumors may be the work of Averroes and, if so, may ultimately derive from Galen, a compilation of whose works Averroes is known to have assembled.

Averroes/Galen? (1126 – 1198 CE /129 – c. 210 CE )

[Treatise on Tumors] .

France: 14 th century.

Terence (c. 190 – c. 160 BCE)

Terentius .

Strassburg: Reinhard Grüninger, 1496.

Both Plautus and Terence, the two Roman comic playwrights whose works survive, make extensive use of stock characters from earlier Greek comedy. These characters frequently include a pair of young lovers and a lena (or procuress), who acts as a go-between. This leaf from a late-15 th -century edition of Terence includes a woodcut of one of the young lovers – the puella , or girl – and the lena from the Hecyra . The first two attempts to stage this play were unsuccessful: on the first occasion the audience abandoned the theater in favor of a tightrope walker and some boxers, and on the second the theater was overrun by spectators from a gladiatorial show.

Hesiod (c. 700 BCE)

Hesiodi Ascraei quae extant .

[Lugduni Batavorum]: Officina Plantiniana, 1603.

PA4009 .A2 1603

Contemporary with the more famous Iliad and Odyssey of Homer, Hesiod’s poetry provides the earliest systematic account in Greek literature of the creation and arrangement of the cosmos, with love playing a central part. In his poem the Theogony , Hesiod names Eros (Love) as one of the very first gods to emerge from primeval Chaos and describes him as “fairest of the immortal gods, loosener of limbs, who overcomes the minds of all gods and mortals”. The Plantin edition of 1603 (left) is a scholarly edition and contains notes and essays by the famous Dutch humanist Daniel Heinsius. The Recurti edition of 1744 (right) contains the first translation of the Theogony into Italian, by Gian Rinaldo Carli. The family trees included in this edition show the central place of Love—“Amore” in the Italian—in Hesiod’s cosmos.

La Teogonia, ovvero la generazione degli dei .

Venezia: Giambatista Recurti, 1744.

PA4010 .I9 T5 1744

Catullus (c. 84 – c. 54 BCE)

Catulli, Tibulli, et Propertii opera .

Birminghamiae: J. Baskerville, 1772.

PA6274 .A2 1772

Although his poetry covers a wide range of topics, Catullus is probably best known for the poems that describe his intense relationship with a woman Catullus calls Lesbia. Perhaps the most famous of these poems is Catullus 85 – Catullus 83 in Baskerville’s edition – a brief but powerful reflection of the contradictions of love: I hate and I love. Why should I do this, perhaps you ask. I do not know; but I feel it happening and I am torn apart . John Baskerville was one of the most influential printers of the eighteenth century. His new, lighter typefaces and his rejection of excessive ornamentation were widely imitated throughout Europe.

Publius Vergilius Maro (70 – 19 BCE)

The Eclogues .

New York: Printed at the press of A. Colish, 1960.

PA6807.B7 C28

Virgil’s Eclogues – also, and perhaps more correctly, called the Bucolics – are the first of his three great contributions to Latin literature, the other two being the Georgics and the Aeneid . Love is a constant theme in the Eclogues and nowhere more that in Eclogue 10, which recounts the fate of Virgil’s fellow poet Gaius Cornelius Gallus. Gallus is imagined wasting away from love, and much of the poem is taken up with a lament spoken by Gallus himself. The lament, and Gallus’ life, ends with the famous line: omnia vincit amor, et nos cedamus amori (‘love conquers all; let me too yield to love’).

Cross-Cultural Methods in Sexual Psychology: Love Research

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• First Online: 08 April 2023
• Cite this living reference work entry

• Marta Kowal 2  

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Culture ; Intercultural research ; Love study ; Romantic love

Cross-cultural methods in research on the sexual psychology of love include varied approaches with the aim to study the phenomenon of love within a broad, representative, cross-cultural perspective.

Introduction

Cross-cultural refers to more than one culture. In research, this term usually applies to inferences taken from data collected from two or more societies. Although love is often considered a universal phenomenon (Buss, 1989 ; Jankowiak & Fischer, 1992 ) with deep biological roots (Shih et al., 2022 ), studies provide evidence that love tends to vary across cultures (Feybesse & Hatfield, 2019 ; Karandashev & Karandashev, 2017 ; Trask, 2021 ). Thus, many scholars have endeavored to shed more light on factors predicting cross-cultural differences in love experiences (see, e.g., Fourmanov, 2021 ; Neto et al., 2000 ). Notably, there are many types of love, including but not limited to: romantic, sexual, parental...

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Kowal, M. (2023). Cross-Cultural Methods in Sexual Psychology: Love Research. In: Shackelford, T.K. (eds) Encyclopedia of Sexual Psychology and Behavior. Springer, Cham. https://doi.org/10.1007/978-3-031-08956-5_734-1

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5 Psychological Theories of Love

Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."

Carly Snyder, MD is a reproductive and perinatal psychiatrist who combines traditional psychiatry with integrative medicine-based treatments.

Anthony Harvie / Stone / Getty Images

Liking vs. Loving

• Color Wheel Model
• Triangular Theory
• Attachment Theory
• Compassionate vs. Passionate

Why do people fall in love ? And why are some forms of love long-lasting while others are so fleeting? Love is a basic human emotion, but understanding how and why it happens is not necessarily easy. Still, many have tried to learn more about this feel-good emotion.

Psychologists and researchers have proposed several different theories of love to explain how it forms as well as how it endures. Here are five of the major theories proposed to explain the psychology of love and other emotional attachments .

In 1970, psychologist Zick Rubin proposed an explanation for the difference between liking and loving . Sometimes we experience a great amount of appreciation and admiration for others. We enjoy spending time with a person and want to be around them. This is "liking," according to Rubin, and doesn't necessarily qualify as love.

Love is much deeper , more intense, and includes a strong desire for physical intimacy and contact. People who are "in like" enjoy each other's company, while those who are "in love" care as much about the other person's needs as they do their own.

Rubin believed that romantic love is made up of three elements:

• A close bond and dependent needs
• A predisposition to help
• Feelings of exclusiveness and absorption

Based on these elements, Rubin devised a questionnaire to assess a person's attitudes toward others. He found that scales ranging from liking to loving provided support for his conception of love.

The Color Wheel Model of Love

In his 1973 book The Colors of Love , psychologist John Lee provided another theory of love, which compared styles of love to the color wheel . Just as there are three primary colors, Lee suggested that there are also three primary styles of love:

• Eros : The term Eros stems from the Greek word meaning "passionate" or "erotic." Lee suggested that this type of love involves both physical and emotional passion. It represents love for an ideal person.
• Ludus :  Ludus comes from the Greek word meaning "game." This form of love is conceived as playful and fun but not necessarily serious. Those who exhibit this form of love are not ready for commitment and are wary of too much intimacy. So, it represents love as a game.
• Storge :  Storge stems from the Greek term meaning "natural affection." This form of love includes familial love between parents and children, siblings, and extended family members. This love can also develop out of friendship, where people who share interests and commitments gradually develop affection for one another. Therefore, it represents love as friendship .

Lee’s 6 Styles of Loving

Lee later proposed that just as the primary colors can be combined to create other colors, the three primary styles of love could also be combined to create secondary love styles. So, in 1977, Lee expanded the list of love styles.

The three new secondary love styles were:

• Mania : A combination of Eros and Ludus , representing obsessive love
• Pragma : A combination of Ludus and Storge , representing realistic and practical love
• Agape : A combination of Eros and Storge , representing selfless love

Triangular Theory of Love

In 1986, psychologist Robert Sternberg proposed the triangular theory of love. Under this theory, love has three components:

Different combinations of these three components result in different types of love. For example, combining intimacy and commitment results in compassionate love while combining passion and intimacy leads to romantic love.

According to Sternberg's triangular theory , relationships built on two or more elements are more enduring than those based on a single component. Sternberg uses the term consummate love to describe combining intimacy, passion, and commitment. While this type of love is the strongest and most enduring, Sternberg suggests that it is also rare.

Attachment Theory of Love

In 1987, Cindy Hazan and Phillip Shaver, two researchers from the University of Denver, shared their views on the psychology of love. They theorized that romantic love is a biosocial process similar to how children form attachments with their parents. Their theory is modeled after psychologist John Bowlby's attachment theory .

According to Hazan and Shaver's attachment theory of love, a person's attachment style is partially formed by the relationship they had with their parents in childhood. This same basic style then continues into adulthood, where it becomes part of their romantic relationships.

The three styles of adult attachment are:

• Anxious/ambivalent : A person with this style often worries that their partner doesn't love them. Sometimes they want to be with their partner so much that it scares the other person away.
• Avoidant : Someone with this style is uncomfortable getting close to others. They also typically experience difficulty with developing trust .
• Secure : As its name suggests, the secure attachment style involves being secure in the relationship. Someone who is secure has very few worries of abandonment or fears of someone else getting too close.

Based on Hazan and Shaver's research, secure attachment is the most common style. This is followed by the avoidant attachment style, then anxious/ambivalent attachment.

Hazan and Shaver also proposed that one's experiences in love and attachment affect their beliefs, which affect their relationship outcomes. It is a cyclical process that can be okay for people with a more secure attachment style but could also create issues for someone who is avoidant or anxious/ambivalent in their relationships.

Compassionate vs. Passionate Love

In 1988, psychologist Elaine Hatfield added to our theories of love by proposing that there are two basic types of love: compassionate love and passionate love .

• Compassionate love is characterized by mutual respect, attachment, affection, and trust. This love usually develops out of feelings of mutual understanding and shared respect for one another.
• Passionate love is characterized by intense emotions, sexual attraction , anxiety, and affection. When these intense emotions are reciprocated, people feel elated and fulfilled, while unreciprocated love leads to feelings of despondency and despair.

Hatfield suggests that passionate love arises when cultural expectations encourage falling in love, when the person meets one's preconceived ideas of ideal love, and when one experiences heightened physiological arousal in the presence of the other person. This love is transitory, according to Hatfield, usually lasting between 6 and 30 months.

Ideally, passionate love leads to compassionate love, which is far more enduring. While most people desire relationships that combine the security and stability of compassionate love with intense passionate love, Hatfield believes that this is rare.

Many theories of love exist, hoping to provide insight into how love forms and evolves. Each one contributes to what we know about this emotion, providing several possible explanations for how love-based relationships begin, grow, and change.

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By Kendra Cherry, MSEd Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."