Between different individuals and cultures, the spectrum of human wants, needs, actions, and responses varies almost infinitely. But despite these differences, one phenomenon seems to persist across the branching history of human cultural evolution. Almost universally, humans are fascinated with the idea of romantic love. Love is not merely a result of modern Western culture, which is saturated with books, shows, and movies about sex and romance. In fact, love is not a result of any one culture’s mainstream ideas. Cross-cultural anthropological studies have repeatedly found that, even if the exact nature of relationships differ between cultures, romantic love as a distinct idea is present in nearly every human population . Given that human culture is so inherently diverse, why and how have similar ideas of love become ubiquitous amongst humans as a whole?
As the science behind both human evolutionary biology and neuroscience have advanced over the last 200 years, explanations behind the phenomenon of love have predictably become more biological. Primarily, this change in perspective was driven by the increased influence of Darwinian logic. If all behaviors are tailored to improve the chances of successful reproduction, it follows that love must merely be the ‘byproduct’ of the chemical motivation to reproduce. According to this line of reasoning, longer partnerships only exist to ensure the survival of potential offspring .
Proponents of this purely scientific perspective would point to ancient pornographic cave paintings and sculptures, arguing that they demonstrate how love is fundamentally derived from sex. Advancements in neurological studies have further unraveled the mystique of romance. Although the precise endocrinology has not yet been defined, it is clear that the hormones testosterone, estrogen, dopamine, oxytocin, and vasopressin all have important roles in creating the phenomenon that we call love . Some could even conclude that the mere presence of similar hormonal pathways in other mammalian species means that human love is nothing special.
What, then, should people make of love? Should we let our understanding of what seems to be irrational and emotional be defined by cold, hard science? Recent research indicates that such a simplistic conclusion may not be the most accurate. In fact, revelations about the human brain in love may help us to better understand the idiosyncrasies of our thoughts, emotions, and behaviors. Even if love itself is nothing more than a chemical process refined over billions of years of evolution, the interconnected neurological intricacies of the human brain and body make the romantic and sexual relationships of Homo sapiens unique among animal species.
Are Sex and Love Products of the Same Phenomenon?
Through a Darwinist lens, it is scientifically logical that love is fundamentally tied to sex. Natural selection dictates that population-wide traits that increase the chances of reproduction become more frequent, and there should be no reason that the behaviors we associate with love would differ. For one, sexual reproduction predates love, and there is evidence that sexual reproduction occurred in algae as far back as 1.2 billion years ago . This finding suggests a possibility that romantic love, which maintains attachment between sexually active couples, has been built from these earliest biological frameworks.
Research into the brains of other mammalian species that have similar neural structures to humans has maintained the long-held belief in the strictly sexual origin of love. One particularly interesting species is the prairie vole, a uniquely monogamous rodent that may be able to act as a biological model for the human brain in love. Unlike most other closely related species, male and female prairie voles form lifelong pair bonds. The pair bonds between voles are enduring and are characterized by “selective affiliation, contact, and mating” with a preferred partner .
These similarities between pair bonding in voles and the social behaviors of humans explain why scientists are so interested in uncovering the circuitry of the vole brain. Like in humans, attachment between paired voles is in part dictated by the effect of dopamine in the nucleus accumbens, a site in the brain that mediates reward behavior and reinforces addictive behavior . In voles, female attachment to a male partner following mating is associated with a 50% increase of dopamine in the nucleus accumbens. In addition, researchers at Emory University discovered that an injection of a dopamine agonist, a drug which enhances the function of dopamine, into the nucleus accumbens is so potent that voles form attachments with any other voles present. Conversely, in the presence of a dopamine antagonist, a drug which blocks the function of dopamine, female voles no longer prefer their existing partners .
Through similar findings in other mammal species, many scientists have concluded that romantic love in humans consists of only a few chemical processes that evolved to maintain sexual relationships. In other words, romantic love would not exist without a long history of sexual evolution. Some recent human studies support this general perspective. In a 2015 study of a Tanzanian hunter-gatherer tribe, Polish researchers found a statistically significant positive correlation between sexual activity within a marriage and the number of children a woman had . Based on their data, the researchers hypothesized that natural selection promoted aspects of love, such as sexual attraction and emotional connection, throughout the course of human evolution.
However, looking at our common human experiences, there seem to be far too many idiosyncrasies in the spectrum of human relationships to conclude that love is only relevant to sexual reproduction. Even if human brains can be compared to our mammalian relatives, it is clear that human love is a far more complex phenomenon than is found in other species. How do scientists resolve the complex nature of human love with the fundamental link between love and sex? Perhaps, what the human mind perceives as the singular phenomenon of love is actually a combination of multiple distinct processes. Even if sex was an essential part of the evolution of love, other supplementary processes may feed into how love manifests in the greater human population today. As research on human relationships continues to expand, it has become more clear that the different aspects of love arise from distinct neurological processes.
How Has New Research Changed Our View of Love?
In the year 1998, biological anthropologist Helen Fisher, in a culmination of years of conducted research, first proposed that romantic love could be broken down into three so-called emotional systems: the sex drive, the attraction system, and the attachment system . Although Fisher clarifies that there are overlaps and subtleties to each of these systems, she hypothesized that they were each driven by their own distinct set of hormones.
According to Fisher, the sex drive, also referred to as libido or lust, is characterized by the desire for sexual gratification. Associated primarily with the estrogen and androgen hormones, the sex drive alone fails to explain monogamous relationships, instead acting only to motivate individuals to copulate with any available partner. The attraction system, termed passionate love or infatuation, is responsible for the focusing of emotional attention towards one individual, particularly at the beginning of a relationship. Because of the seemingly subconscious nature of infatuation, the attraction system is tied to the reward system of the brain, which involves the neurotransmitters dopamine, norepinephrine, and serotonin. These neurotransmitters act in the prefrontal cortex and the nucleus accumbens to motivate humans towards rewarding behaviors . Finally, the attachment system, deemed “companionate love,” is involved in romantic love, but also allows non-romantic relationships in the form of friendships, parent-infant bonding, and other social affiliations. The attachment system is dictated by the hormones oxytocin and arginine vasopressin, synthesized in the hypothalamus.
Through the articulation of these three systems, Fisher explains why an individual can express deep attachment to a long-term spouse, become attracted to someone else, and lust for distant individuals all at the same time. These systems also explain how individuals can maintain lifelong platonic relationships, without the necessity of a sexual relationship . As a whole, Fisher proposed that a singular view of love is outdated, and she paved the way for a paradigm shift in the way biologists, anthropologists, and neuroscientists all approach the topic of love.
Most of Fisher’s initial proposals have been supported and built upon by further research. More recent work has primarily focused less on the sex drive system, and more on the attraction and attachment systems which are generally less well-understood and thus of more interest to many scientists. Altogether, the connections between these three systems have become more clear. In a study of female prairie voles at Michigan State University, female voles placed in the presence of male vole urine released norepinephrine, a hormone integral to the attraction system, into specific areas of the olfactory bulb of the brain, stimulating the release of estrogen . In other words, the attraction system leads to the immediate strengthening of sexual desire and behavior. However, in humans, the nature of the relationship between attraction and the sex drive is not so clear. When middle-aged men and women are treated with testosterone, their sex drive increases, but they do not fall in love, indicating that the two systems are not as closely attached in humans . Regardless, further research into humans has reaffirmed the association of attraction with dopamine release, providing evidence for the ‘addictive’ nature of love .
Research has shown similar close links between the sex drive and the attachment system. Oxytocin and vasopressin, the key hormones of the attachment system, have been found to interact with the biochemical pathways of the other two systems . In mammals, sex hormones like estrogen stimulate oxytocin release from neurons in the hypothalamus and increase the binding of oxytocin to its receptors in the amygdala, which regulates the processing of emotion. One effect of the interaction between oxytocin and the amygdala is decreased levels of anxiety. For the attachment system, this calming effect is essential in linking sexual activity and the formation of emotional attachment. Oxytocin is also a crucial part of the attraction system, as it operates through neurons that release serotonin, a neurotransmitter that helps improve mood and relieve anxiety. Additionally, the hypothalamic cells that release oxytocin are activated by dopamine receptors, and the pathways of dopamine and oxytocin converge in the prefrontal cortex, the part of the brain responsible for complex behavior and decision making . The ties between oxytocin and the attraction system are still being investigated, but these initial links hint at biological mechanisms for the interaction between the three systems in the formation of relationships, both romantic and platonic.
Further Idiosyncrasies of the Human Brain in Love
Even under Fisher’s three-pronged framework for love, much of the diversity in human romantic activity remains unexplained. For instance, how does homosexuality, which was not addressed by the previous hypotheses, inform our understanding of the nature of the brain? For many years, homosexuality, as well as other divergent relational or sexual behaviors, had been considered the sole result of social experiences during early childhood . But as our ability to survey the brain has increased, researchers have uncovered neurobiological correlates of homosexuality. These findings have broader implications about what parts of the brain are involved in love in general.
At the Karolinska Institute in Sweden, imaging research on homosexual individuals led by neuroscientist Ivanka Savic uncovered evidence for structural and functional brain differences related to gender and sexual orientation . Having observed sexual dimorphism in human brains, Savic hypothesized that homosexuals would exhibit brain structures more characteristic of the other sex. Using magnetic resonance imaging scans, Savic revealed that heterosexual men and homosexual women demonstrated a volumetric rightward cerebral asymmetry, whereas homosexual men and heterosexual women demonstrated a leftward cerebral asymmetry . Savic’s group found a similar pattern in the amygdala, where homosexual individuals showed sex-atypical connections. Although Savic’s findings represent only a small component in the study of human sexual orientation, her research has helped to shape our “map” of the parts of the brain involved in love, both heterosexual and homosexual.
Before Savic’s findings, brain features like volumetry, the connectivity of the amygdala, and other amygdala-adjacent structures were thought to be unrelated to reproduction. But because these features differ between individuals with different sexual orientations, scientists believe that they may play a role in the biochemical pathways of the three components of love. Studies conducted by other research groups have yielded similar results. At the University of Amsterdam, the suprachiasmatic nucleus (SCN), which is thought to regulate circadian rhythms and hormone levels, was found to be twice the size in homosexual males than in heterosexual males . In a follow-up experiment on rats, the researchers induced similar changes by inhibiting interactions between testosterone and the developing brain. These researchers found that the experimental group of rats with larger SCNs also had a larger number of vasopressin neurons in their SCNs. Because testosterone and vasopressin are involved in sex drive and the attachment system respectively, the SCN is now thought to play a role in both systems. Thus, by comparing the properties of a brain region in different groups of people, scientists can draw conclusions about the general function of those regions.
Fisher’s research also raises questions about the extent to which existing variances in brain structure affect relationship patterns of individuals. Would individuals born with similar brain structures have the same patterns of monogamy and sexual activity? Do differences among individuals in brain activation indicate fundamental differences, or are they conditioned as a result of choices made over a lifetime? Two recent studies are characteristic of the many ongoing attempts to find the neurological cause for behavioral differences expressed over a lifetime.
In a 2017 study at the University of Texas at Austin, researchers brought twenty sexually active heterosexual men and placed them in a fMRI machine . Ten of these men were classified as highly monogamous, while the other ten reported frequent non-monogamous activity. Each man was shown three different types of images — sexual, romantic, and neutral — while researchers observed the patterns in their brain activation. Unsurprisingly, researchers found no differences between the two groups in brain activation when shown the sexual images. However, when shown the romantic images, the monogamous men tended to have increased neural activity in the nucleus accumbens, the prefrontal cortex, and other reward-related regions in the brain as compared to the non-monogamous men. These regions, which have high densities of vasopressin and dopamine receptors, are essential to the attachment system. While the researchers hypothesize that relationship experiences may cause some of the differences in brain activity, the findings indicate that biological differences at birth may be correlated with predispositions to monogamy and nonmonogamy.
A collaborative study involving both Australian and Finnish researchers attempted to answer the question of monogamy through a different lens . Given data from more than 7000 Finnish twins and siblings, researchers attempted to explain monogamy rates through genetic analysis. Following a survey of the twins and siblings, the researchers found that genetic factors contributed to 63% of the variation in infidelity in men and 40% in women. But while the researchers solidified the basis for a genetic component to behavior in romantic relationships, they also raised new uncertainties within our existing framework for understanding love. Because vole monogamy is almost entirely dependent on the density of oxytocin and vasopressin receptors, it was long assumed that variations in these receptors would have similar importances in humans. But in this study, researchers found only a small association between the arginine vasopressin receptor 1A (AVPR1A) gene and infidelity, and no association for the oxytocin receptor (OXTR) gene at all. While these hormones still remain essential to the way the human brain processes love, the findings of this study suggest that we are a long way from fully understanding the exact neurological mechanisms of love.
In a 2018 interview, Harvard Medical School professor of psychiatry Richard Schwartz concluded, “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” . The wide range of research done in the field of love has done little more than classify processes and emotions that many people already intuitively understand. That is not to say that all the research has been useless. Our improved understanding of the biochemical pathways is likely to yield translational results in medical treatment practices. For example, comparisons between love and addiction have led to the experimental use of the sedative propofol as a way to relieve emotionally painful memories . Additionally, the trust-inducing properties of oxytocin have been found in some studies to help those with social anxiety, depression, and even autism . But until future research draws stronger links between human behaviors and brain functions, a scientific understanding of love may be more useful in the lab than in real life.
 Jankowiak, W., & Fischer, E. (1992). A Cross-Cultural Perspective on Romantic Love. Ethnology, 31(2), 149-155. doi:10.2307/3773618.
 Haufe, C. Sexual selection and mate choice in evolutionary psychology. Biol Philos 23, 115–128 (2008). https://doi.org/10.1007/s10539-007-9071-0.
 Fisher, H. E., Aron, A., & Brown, L. L. (2006). Romantic love: a mammalian brain system for mate choice. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 361(1476), 2173–2186. https://doi.org/10.1098/rstb.2006.1938.
 de Boer, A., van Buel, E. M., & Ter Horst, G. J. (2012). Love is more than just a kiss: a neurobiological perspective on love and affection. Neuroscience, 201, 114–124. https://doi.org/10.1016/j.neuroscience.2011.11.017.
 Sorokowski P, Sorokowska A, Butovskaya M, et al. Love Influences Reproductive Success in Humans. Front Psychol. 2017;8:1922. Published 2017 Nov 21. doi:10.3389/fpsyg.2017.01922.
 Fisher, H. E., Xu, X., Aron, A., & Brown, L. L. (2016). Intense, Passionate, Romantic Love: A Natural Addiction? How the Fields That Investigate Romance and Substance Abuse Can Inform Each Other. Frontiers in psychology, 7, 687. https://doi.org/10.3389/fpsyg.2016.00687.
 Fisher, H.E. Lust, attraction, and attachment in mammalian reproduction. Hum Nat 9, 23–52 (1998). https://doi.org/10.1007/s12110-998-1010-5.
 Smale, L., Nelson, R. J., & Zucker, I. (1985). Neuroendocrine responsiveness to oestradiol and male urine in neonatally androgenized prairie voles (Microtus ochrogaster). Journal of reproduction and fertility, 74(2), 491–496. https://doi.org/10.1530/jrf.0.0740491.
 Earp, B. D., Wudarczyk, O. A., Foddy, B., & Savulescu, J. (2017). Addicted to love: What is love addiction and when should it be treated?. Philosophy, psychiatry, & psychology : PPP, 24(1), 77–92. https://doi.org/10.1353/ppp.2017.001.
 Meyer-Lindenberg, A., Domes, G., Kirsch, P. et al. Oxytocin and vasopressin in the human brain: social neuropeptides for translational medicine. Nat Rev Neurosci 12, 524–538 (2011). https://doi.org/10.1038/nrn3044.
 Swaab D. F. (2008). Sexual orientation and its basis in brain structure and function. Proceedings of the National Academy of Sciences of the United States of America, 105(30), 10273–10274. https://doi.org/10.1073/pnas.0805542105.
 Savic, I., & Lindström, P. (2008). PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects. Proceedings of the National Academy of Sciences of the United States of America, 105(27), 9403–9408. https://doi.org/10.1073/pnas.0801566105.
 Zuoxin Wang, Brandon J. Aragona, Neurochemical regulation of pair bonding in male prairie voles, Physiology & Behavior, Volume 83, Issue 2, 2004, Pages 319-328, ISSN 0031-9384, https://doi.org/10.1016/j.physbeh.2004.08.024.
 Ventura, Rossella & Morrone, Cristina & Puglisi-Allegra, Stefano. (2007). Prefrontal/accumbal catecholamine system determines motivational salience attribution to both reward- and aversion-related stimuli. Proceedings of the National Academy of Sciences of the United States of America. 104. 5181-6. 10.1073/pnas.0610178104.
 Hamilton, L.D., Meston, C.M. Differences in Neural Response to Romantic Stimuli in Monogamous and Non-Monogamous Men. Arch Sex Behav 46, 2289–2299 (2017). https://doi.org/10.1007/s10508-017-1071-9.
 Zietsch, Brendan & Westberg, Lars & Santtila, Pekka & Jern, Patrick. (2014). Genetic analysis of human extrapair mating: Heritability, between-sex correlation, and receptor genes for vasopressin and oxytocin. Evolution and Human Behavior. 36. 10.1016/j.evolhumbehav.2014.10.001.
 Vallejo, Ana & Kroes, Marijn & Rey, Enrique & Acedo, Maria & Moratti, Stephan & Fernández, Guillén & Strange, Bryan. (2019). Propofol-induced deep sedation reduces emotional episodic memory reconsolidation in humans. Science Advances. 5. eaav3801. 10.1126/sciadv.aav3801.
 Bakermans-Kranenburg, M. J., & van I Jzendoorn, M. H. (2013). Sniffing around oxytocin: review and meta-analyses of trials in healthy and clinical groups with implications for pharmacotherapy. Translational psychiatry, 3(5), e258. https://doi.org/10.1038/tp.2013.34.
 Gibson, Timothy & Shih, Patrick & Cumming, Vivien & Fischer, Woodward & Crockford, Peter & Hodgskiss, Malcolm & Wörndle, Sarah & Creaser, Robert & Rainbird, Robert & Skulski, Thomas & Halverson, Galen. (2018). Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis. Geology. 46. 135-138. 10.1130/G39829.1.
 Young, K. A., Gobrogge, K. L., Liu, Y., & Wang, Z. (2011). The neurobiology of pair bonding: insights from a socially monogamous rodent. Frontiers in neuroendocrinology, 32(1), 53–69. https://doi.org/10.1016/j.yfrne.2010.07.006.
 Powell, A. (2019, January 23). Scientists find a few surprises in their study of love. Retrieved March 01, 2021, from https://news.harvard.edu/gazette/story/2018/02/scientists-find-a-few-surprises-in-their-study-of-love/.