Homosexual Behavior Evolved to Bring People Closer Together

For decades, scientists have speculated that homosexual behavior might have something to do with affiliation, the desire to forge and maintain social bonds. But these findings have been observational until now. We’ve conducted the first research which finds that the desire to interact sexually with people of the same sex might just be another way of being friendly.

Erotic interactions between people of the same sex are very common, and much (if not most) homosexual behavior occurs among people who don’t identify as exclusively homosexual. One evening a friend told me that she’d ended up kissing one of her best friends after they shared an especially vulnerable moment. Young men seem to kiss each other pretty often, too: One interview study found that 89 percent of young heterosexual British men reported having kissed another man on the lips. And homosexual behavior alongside heterosexual behavior isn’t limited to our species or Western culture. Another study from the 1950s showed that a majority of cultures exhibited some homosexual behavior, and that 64 percent of cultures considered it acceptable. Homosexual behavior is also very common in our primate relatives. Bonobos, otherwise known as pygmy chimpanzees, famously use sex to negotiate food sharing and to make up after fights. Alliance-promoting homosexual behavior has also been found in monkeys, baboons and gorillas.

If sex is such a good way of helping two people to get along, why should it be limited to people of the opposite sex?

Why does sex feel good?

Our biological systems encourage us to engage in sexual behavior with delicious feelings of pleasure, sweet feelings of closeness, and sometimes fierce motivation, all because it’s the best way to get our genes into the next generation. But, you may have noticed, it’s not just the sex that can make babies that feels good, and people are often attracted to people they can’t reproduce with. If you were to ask almost anyone why couples have sex when there is no possibility of pregnancy, they’d probably tell you it helps maintain the bond that’s going to keep them liking each other across time and through difficult situations. If sex is such a good way of helping two people to get along, why should it be limited to people of the opposite sex?

To investigate whether or not affiliation might have something to do with homosexuality, we first looked at progesterone. Progesterone is produced in both men and women; studies have found that it’s closely tied to the drive to affiliate but, importantly, not positively associated with an increase in sexual desire. Progesterone has also been shown to increase in both men and women when they are socially rejected and then given an opportunity to meet new people. Another study found that women randomly paired together to do a task of social closeness showed increased progesterone levels; this increase predicted how willing each woman would be to sacrifice for her task partner one week later.

We asked women to come into the lab and collected saliva samples. Then we asked them a few questions intended to get at whether or not they were motivated to be sexual with other women, including “The idea of kissing a woman seems sexually arousing to me,” and “Women’s bodies are erotic.” We found that women with higher progesterone levels were more likely to say that they would have erotic contact with other women.

©Diana S. Fleischman/Archives of Sexual Behavior

If it is the case that we have evolved to have motivation for sexual contact for both reproductive and affiliative reasons, it would make sense that there are tradeoffs between these two motivations. One example of this kind of tradeoff is that in most mammals, females eat fewer calories when they are ovulating but travel greater distances. Evolution may have crafted this so that the motivation to eat did not compete with the motivation to seek out mates.

Similarly, the motivation to engage in same-sex sexual affiliation might be reduced when there are reproductive opportunities. Although we didn’t find a significant effect, we did find a pattern indicating that during ovulation women may be less motivated to engage in homoerotic behavior.

©Diana S. Fleischman/Archives of Sexual Behavior

After this study, we wanted to see if progesterone and the motivation to affiliate had the same effect on men. First we brought men into the lab and got saliva samples. Then we randomly assigned them one of three conditions: affiliative, opposite-sex sexual, and neutral. We used word puzzles that involved filling in blank letters to get participants into a certain state of mind. For example, in the sexual condition, men would have filled out BR_ _ _ TS (Breasts) and in the affiliative condition would have filled out FRI _ _ NDS (Friends).

We found not only that men in the affiliative condition showed higher scores on the measure of homoerotic motivation, but also that progesterone interacted significantly with the priming condition, such that men in the affiliative condition with high progesterone showed the highest scores on homoerotic motivation. Sexual primes, on the other hand, did not increase homoerotic motivation, perhaps because they indicated that reproductive success was a possibility and thus affiliative homoerotic interactions might not be the most adaptive at that time.

©Diana S. Fleischman/Archives of Sexual Behavior

Is there a gay gene?

These results have some intriguing implications. One question that might arise is, “What does this research mean for people who are exclusively homosexual?” Many hypotheses have been offered about why exclusive homosexual orientation might exist. For example, a few studies have shown that the female relatives of gay men are more fertile; thus homosexuality might be a byproduct of genes for increased female fertility. Our research suggests that if the motivation to engage sexually with others of the same sex is adaptive and affiliative, it is unlikely there will be a “gay gene,” since adaptive traits tend to be regulated by multiple genes, and there will be variability in how much this trait is expressed.

Like any trait that is maintained in the population, there will be people who fall on either extreme of the continuum from exclusively homosexual to exclusively heterosexual. In societies where even affectionate contact between members of the same sex is culturally repressed, we may not see the full range of affiliative same-sex interactions. But, as we are already seeing, as cultural norms change, so too will the expression of the full diversity of affiliative same-sex interactions.