Why do we love?
Human romantic love is a puzzling anomaly. Long-term monogamous relationships are exceedingly rare among animals; indeed, less than 3% of mammals form monogamous relationships. So why are we the exception? The dominant theory holds that love is an evolved adaptation that contributed to our survival as species. The shift to walking upright changed our ancestors’ pelvic structure in a way that narrowed the birth canal in females, and necessitated that newborns be very small. Human babies are extremely helpless at birth and take a long time to mature sexually, so they benefit greatly from the continuous care and dedication of both parents. Love, in short, is a commitment mechanism that encourages the mom and dad to stay together and raise a healthy, sexually mature individual that can successfully pass on their genes1. So if you sometimes feel like a fool for love, remember that it’s in your nature.
What is love?
The ultimate answer to this question may vary across time, cultures, and individuals, but at the biological level love is, arguably, oxytocin. The hottest thing in love research, oxytocin is a brain
chemical that plays an important role in social relationships, especially parental attachment and pair bonding. It floods females' brains during childbirth andbreast-feeding, contributing to mother-child bonding. In males, an oxytocin-like chemical called vasopressin is linked to relationship quality, and is implicated in paternal investment and monogamy. Dubbed the 'cuddle hormone', oxytocin is also released by both sexes during intimacy and orgasm. It has been shown to increase trust, empathy, and generosity towards strangers, lower anxiety and stress, facilitate social recognition, dampen fear, enhance social functioning for various mental disorders, and, remarkably, speed up wound healing2-5.
However, the effects of oxytocin depend on the social context and are far from fully understood. For example, in one study oxytocin’s ability to dampen anxiety in response to a stressor interacted with how much social support was available to the study participant6. This ‘love potion’ may also have negative effects. In laboratory mice, administrations of oxytocin increase aggression. In humans, a recent study showed that participants who were given oxytocin experienced more envy and gloating towards a fellow participant, which suggests that oxytocin might simply strengthen social emotions, positive or negative. Despite the uncertainty surrounding its effects, we do know that this chemical functions in the brain’s reward centers. We appear wired to experience attachment as pleasurable. In fact, it is so pleasurable that the elation of a new love, in a way, mimics the effects of certain drugs or a manic episode7. But a new love is different from an old and familiar one, and your brain knows the difference.
Are love and passion the same?
Most people believe that the feeling of “falling in love” is different from that of long-term attachment and, at a biological level, they are absolutely correct. If love is oxytocin, passion is dopamine. Like heroine, cocaine, and other addictive drugs, passion increases the circulation of this well-known ‘pleasure chemical’ in the reward circuits of the brain7. This raises the intriguing possibility that people can become addicted to falling in love and that the pain of romantic rejection is akin to that of drug withdrawal. Passion also increases norepinephrine (a hormone similar to adrenaline), which narrows attention, increases heart rate, and fosters goal-oriented behavior. Remarkably, passion is also associated with lower levels of another important neurotransmitter—serotonin. Low levels of this chemical are also found in people with obsessive-compulsive disorder. So next time your friend can’t stop talking about their new crush, you can blame it on norepinephrine. In addition, some anti-depressants, such as Prozac, artificially boost serotonin levels, so some researchers worry that such drugs may actually prevent those who use them from falling in love8.
Unlike passion, long-term romantic attachment is associated with the release of oxytocin and endorphins, the body's natural painkillers. Aside from promoting trust and bonding, these chemicals induce a sense of calm and peaceful contentment and are involved in the formation of habits. Love, in this sense, is a lot like driving an old Volvo. Curiously, a small percentage of couples who’ve been married for many years report that they still feel very much in love, passionate as the day they met. Brain scans of such couples reveal that indeed the chemicals associated with passion are present at levels similar to those in the brains of people who’ve recently fallen in love9. They’re still riding a red hot convertible and I must admit that I don’t need a dose of oxytocin to envy them.
What is physically attractive?
Despite a common perception that beauty standards vary widely around the world, studies have shown that people from diverse cultures tend to agree on which faces are hot and which are not (most research in this area focuses on facial rather than body shape attractiveness)10. In fact, even infants prefer to look at attractive faces over non-attractive ones, which suggests that perceptions of beauty are fundamental and universal11. This supports evolutionary theories according to which attractive characteristics are those which draw us to healthy and reliable partners who can provide quality genes and care to potential offspring. So if beauty is not exactly in the eye of the beholder, then where is it?
Two of the best-studied predictors of physical attractiveness are symmetry and averageness12. When people are asked to rate pictures of normal faces along with computer-generated symmetrical versions of the same faces, almost everyone views the symmetrical images as more attractive. Similarly, when researchers use software to blend the faces of multiple people into a single composite (whose features are thus more average or prototypical), that ‘averaged’ face is rated as more attractive than the ones used to create it. Psychologists at the University of Aberdeen maintain a website that demonstrates the symmetry and averageness effects. You can even upload your own images and see what you might have looked like if you were blessed with perfectly symmetrical or prototypical features (although, in my opinion, this exercise can be a bit depressing).
A third major attractiveness booster is sexual dimorphism, i.e. having exaggerated feminine/masculine traits (e.g. small chin, full lips, large eyes, and high cheekbones in females, and small eyes, prominent eyebrow ridge, and wide jaw in males). It has been suggested that human preferences for symmetry, averageness, and sexual dimorphism have evolved because such characteristics signal a mate’s high genetic quality and resistance to diseases, and thus overall health. However, research with humans shows only weak and inconsistent associations between facial attractiveness and health12. Specifically, averageness and masculinity in males is weakly associated with health, whereas averageness and femininity in females, and symmetry in both sexes appear unrelated to health12.
Aside from facial features, smell and taste may also affect how much ‘chemistry’ we have with someone. Several studies have shown that women are attracted to the scents of men whose immunity genes are most dissimilar to their own13,14. Such a preference may have evolved because children whose parents carry different immunity genes are likely to be resistant to a greater number of diseases. Men too may pick up subtle odor cues in assessing females' attractiveness. Recent studies suggest that men prefer display higher testosterone levels in response to the natural scents of women who are ovulating (and who are, therefore, most likely to conceive)15,16. As for taste, it appears that saliva also contains information about an individual’s immunity genes17. Perhaps kissing is then nature’s taste test!
And cheating on nature’s tests might carry unexpected costs. Unlike normally-ovulating women, those who take oral contraceptives prefer the scents of men with similar immunity genes18. They also do not show a preference for masculine and symmetrical male faces. In light of such findings, researchers are voicing concerns that oral contraceptives may interfere with a woman’s evolved sensitivity to cues of a mate’s genetic quality and compatibility, as well as her natural appeal to men18. There have even been speculations that the rising divorce rates of recent decades may in part reflect the increased use of oral contraceptives. Research on this topic is in its infancy, but I advise all women who are on the Pill to follow this literature closely.
Is that all there is to it?
Even though we can’t do much in the way of improving our symmetry, averageness, smell, or taste, there is fortunately plenty more to being desirable in the long run. Our appeal can and does depend on our expressions, personality, and behavior. For example, one study found that a smiling face was rated as more attractive than the same face with a neutral expression.
However, this was only true when the smiling person was gazing ahead, i.e. at the viewer; when the face was smiling but looking sideways, the neutral expression was rated more favorably19. So if you want to attract someone be sure to smile and look at them, and perhaps avoid smiling at others.
In addition, sometimes just ‘being around’ can help. The so called “mere exposure effect” is a well-established psychological phenomenon which shows that the more we're exposed to a stimulus the more we like it. This has been shown to hold true for everything from people, to music, to meaningless alphabetical characters. And lastly, and perhaps most obviously, who you are matters! One study showed that across more than 30 cultures the top 3 most desirable characteristics in a mate were warmth/kindness, intelligence, and an interesting personality20.
And this is probably the best advice you can give your dinner audience. There are many people with attractive features and scents out there, but if you want to be liked and loved in the long run, being a good and interesting person is probably your best bet.
1. Gonzaga, G., Turner, R., Keltner, D., Campos, B.,&Altemus, M. Romantic love and sexual desire in close relationships. Emotion 6, 163-179 (2006).
2. MacDonald, K.&MacDonald, T. The Peptide That Binds: A systematic review of oxytocin and its prosocial effects in humans. Harvard Review of Psychiatry 18, 1–21 (2010).
3. Lee, H-J., Macbeth, A., Pagani, J.,&Young, S. Oxytocin: The great facilitator of life. Progress in Neurobiology 88, 127–15 (2009).
4. Kosfeld, M., Heinrichs, M., Zak, P., Fischbacher, U.&Fehr, E. Oxytocin increases trust in humans.Nature 435, 673-676 (2005).
5. Gouin, J-P., Carter, C., Pournajafi-Nazarloo, H., Glaser, R., Malarkey, W., Loving, T., Stowell, J.,&Kiecolt-Glaser, J. Marital behavior, oxytocin, vasopressin, and wound healing.Psychoneuroendocrinology (2010), doi:10.1016/j.psyneuen.2010.01.009.
6. Heinrichs, M., Baumgartner, T., Kirschbaum, C., Ehlert, U. Social support and oxytocin interact to suppress cortisol and subjective responses to psychosocial stress. Biological Psychiatry 54, 1389-1398 (2003).
7. Young. L. Love: Neuroscience reveals all. Nature 457, 148 (2009).
8. Fisher, H.&Thomson, J. A. in Evolutionary Cognitive Neuroscience (eds. Platek, S., Keenan, J.&Shakelford, T.) 245-283 (MIT Press, Cambridge, MA, 2007)
9. Aceveda, B., Aron, A., Fisher, H.&Brown, L. Neural correlates of long-term pair-bonding in a sample of intensely in-love humans. Annual Meeting of the Society for Neuroscience, Poster Session#297, (2008).
10. Rhodes, G., Yoshikawa, S., Clark, A., Lee, K., McKay, R.&Akamatsu, S. Attractiveness of facial averageness and symmetry in non-western populations: in search of biologically based standards of beauty. Perception 30, 611–625 (2001).
11. Langlois, J. H., Ritter, J. M., Roggman, L. A.,&Vaughn, L. S. Facial diversity and infant preferences for attractive faces. Developmental Psychology 27, 79-84 (1991).
12. Rhodes, G. The evolutionary psychology of facial beauty. Annual Reviews in Psychology 57, 199-226 (2006).
13. Wedekind C, Seebeck T, Bettens F, Paepke A.J. MHC-dependent mate preferences in humans.Proceedings of the Royal Society of Biological Sciences 260, 245–249 (1995).
14. Havlicek, J. and Roberts, C. MHC-correlated mate choice in humans: A review.Psychoneuroendocrinology 34, 497–512 (2009).
15. Miller, S.&Maner, J. Scent of a Woman. Psychological Science 21, 276–283 (2010).
16. Miller, G., Tybur, J.,&Jordan, B. Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus? Evolution and Human Behavior 28, 375–381 (2007).
17. Kluger, J. The science of romance: Why we love. Time Magazine (Jan. 17, 2008).
18. Alvergne, A.&Lummaa, V. Does the contraceptive pill alter mate choice in humans? Trends in Ecology and Evolution 25, 171-179 (2010).
19. Jones, B. C., DeBruine, L. M., Little, A. C., Conway, C. A. & Feinberg, D. R. Integrating physical gaze direction and expression with physical attractiveness when forming face preferences.Psychological Science 17, 588-591 (2006).
20. Buss, D. Sex differences in human mate preferences: Evolutionary hypotheses tested in 37 cultures. Behavioral and Brain Sciences 12, 1-49 (1989).