A lot of people argue that homosexuality doesn’t make sense from an evolutionary perspective. If humans have an intrinsic motivation to pass their genes on to future generations, homosexuality would make it, well, challenging to accomplish that. This begs the question of why: Why has homosexuality existed throughout human history? And if it inhibits the ability to reproduce, why does this trait persist in the population instead of disappearing? We don’t know the answers to these questions for sure, but scientists have a few ideas.


HOMOSEXUALITY MAY BE ADAPTIVE (IN A NON-OBVIOUS WAY)
Some researchers think that homosexuality does have an adaptive purpose after all—a purpose that might not be immediately apparent to the casual observer.

Enter the kin selection hypothesis, or as it’s known colloquially, the “gay uncle hypothesis.” The basic idea behind it is that gay men supposedly compensate for the fact that they cannot reproduce (at least not without the aid of modern technology) by investing in the offspring of their siblings. For example, they might participate in babysitting or help pay for college. In doing so, they would ensure that at least some of their genes get passed along.

Although this theory has intuitive appeal, we haven’t seen much scientific support for it, at least not in Western cultures like the United States. In American studies, we see that, compared to heterosexual people who don’t have children of their own, gay men without children aren’t necessarily any more likely to help out their nieces and nephews.

There appears to be multiple “kinds” of homosexuality that have different biological roots.

Interestingly, though, researchers have found support for this theory in cultures where men with same-sex attraction don’t tend to identify as gay, but rather as a third gender. For example, consider the fa’afafine (a term that means “in the manner of woman”) on the island of Samoa. These are biological males who take on feminine gender roles and who are attracted to other men—men who consider themselves to be straight. Research has found that the fa’afafine nurture their nieces and nephews, and much more so compared to the straight, childless men in this culture.

So why do we see support for this theory in a place like Samoa but not in the United States? Perhaps because the fa’afafine are widely accepted in Samoa, whereas in the U.S. and other cultures, men with same-sex attraction are frequently disowned from their families. Think about it this way: it’s hard to help your relatives when they don’t want anything to do with you. In other words, maybe male homosexuality only serves this adaptive function in cultures where it is accepted.

HOMOSEXUALITY MAY LIMIT CONFLICT Kin selection isn’t the only theory to suggest that same-sex attraction might have an adaptive purpose, though—and this theory is obviously limited in that it’s only focused on men. What about female same-sex attraction?

A different theory—one claiming that both male and female same-sex attraction are adaptive—is the social glue hypothesis. The thought here is basically that same-sex behavior helps humans to create and maintain social bonds with others. In other words, maybe same-sex attraction and behavior evolved as a way of helping to reduce conflict and form alliances.

Research suggests that same-sex behavior among non-human primates is commonly practiced for these purposes. Though little research has addressed whether the same is true of humans, one study found that, for both men and women, higher levels of the hormone progesterone—a hormone known to play a role in social bonding—are linked to holding more favorable attitudes toward sex with a same-sex partner.

OR PERHAPS HOMOSEXUALITY IS A BYPRODUCT
Evolutionary researchers make a distinction between adaptations, or traits that evolve in order to help us solve certain problems, and byproducts, or traits that happen to go along with adaptations but that don’t actually solve problems in and of themselves. Some scientists have argued that perhaps instead of representing the former, maybe homosexuality represents the latter. That is, maybe homosexuality is nothing but a byproduct of some other adaptive trait that evolved.

The most popular thought here is that homosexuality in men is a byproduct of genes that are linked to high fertility in women. Supporting this idea, scientists have found that, on their mother’s side of the family, gay men’s female relatives have more offspring compared to straight men’s female relatives.

Even if these genes increase the odds of women having some sons who are gay (and who, therefore, probably won’t reproduce), the overall increase in their fertility would more than make up for that. This theory is intriguing because it offers a plausible way of explaining why homosexuality persists instead of disappearing.

With all of that said, we cannot say for sure which, if any, of these theories are correct. It could also be that more than one theory is right, given emerging evidence that there’s no such thing as a single “gay gene” shared by all gay people; instead, it’s possible that multiple “kinds” of homosexuality, with different biological roots, may exist. This suggests the provocative and fascinating possibility there might even be different evolutionary explanations for different kinds of homosexuality and that maybe—just maybe—some forms of homosexuality are adaptations while others are byproducts.


Justin Lehmiller, PhD is a sex educator and researcher at Ball State University, a Faculty Affiliate of The Kinsey Institute, and author of the blog Sex and Psychology. Follow him on Twitter @JustinLehmiller.