The Evolution of Love and Bonding

Although in our initial study we did not find substantial differences between men and women in the frequency with which they had sex for emotional bonding reasons, a study conducted in the Buss Lab showed a big gender difference in the importance placed on emotional connectedness with a sexual partner. In the study, heterosexual men and women from many different countries were asked a provocative question:

Please think of a serious or committed romantic relationship that you have had in the past, that you currently have, or that you would like to have. Imagine that you discover that the person with whom you’ve that you discover that the person with whom you’ve been seriously involved became interested in someone else. What would upset or distress you more: a) imagining your partner forming a deep emotional attachment to that person or b) imagining your partner enjoying passionate sexual intercourse with that other person?

Hands down, women were more distressed by thinking about their partners being emotionally attached to someone else than by thinking about their partner having sex with someone else. This makes perfect evolutionary sense. From a woman’s viewpoint, a man having sex with another woman may or may not mean that he is emotionally attached to her—it could simply involve physical gratification. But if a man is emotionally attached to another woman, there is a good chance he is (or will soon be) also having sex with her. If a man is both emotionally attached to and having sex with another woman, there is a high probability that he will begin to reallocate his commitment and resources to her instead of to his current partner—a clear threat, in evolutionary terms.

It is worth pausing to recall that, despite our earlier example of prairie voles, sex for the vast majority of species on this planet involves no commitment whatsoever. Humans are the rare exceptions, even among primates, in being one of the few species in which males and females form long-term pair-bonds that last years, decades, and sometimes a lifetime. Among chimpanzees, the primates that are genetically closest to humans, sex occurs primarily when a female enters estrus. During this period of ovulation, the female chimpanzee’s bright red genital swelling and scents send males into a sexual frenzy, but outside of estrus, male chimps are largely indifferent to females. Thus, sexual relationships among chimpanzees are short-lived. Among humans, ovulation is concealed or cryptic, at least for the most part. Although there may be subtle physical changes in women—a slight glowing of the skin or an increase in women’s sexual desire—there is little scientific evidence that men can reliably detect when women are ovulating. From an evolutionary standpoint, successful ancestral men typically would have needed to stick around to have sex throughout a woman’s menstrual cycle. Without cues to ovulation, a single act of sex results in conception only 3 to 4 percent of the time. Stopping by for an afternoon romp rarely paid reproductive dividends. For this reason, some researchers believe that concealed ovulation probably evolved as a way to increase pair-bonding or commitment in human sexual relationships. This in turn increased the chances that resources would be allocated to a single mate and her children.

But that doesn’t explain the powerful emotion of love and why it evolved in humans. Evolutionary psychologists believe that it may be a form of “long-term commitment insurance.” If your partner were blinded by an uncontrollable emotion that could not be helped or chosen, an emotion that was elicited only by you and no other, and one that was made all the more powerful by its association with a cascade of sex-triggered hormones, then commitment would be less likely to waver in sickness as well as in health and if poorer rather than richer. On the other hand, if a partner chooses you based on mostly “rational” criteria—say, your access to resources or your lack of resource-eating offspring—he or she might leave you on the same basis—in favor of a competitor with slightly more desirable qualities.