By David P. Barash
The Chronicle of Higher Education, Nov 19, 2012 

The mystery of gayness: Homosexuals reproduce less than heterosexuals, so why has natural selection not operated against it?

Here are some promising possibilities.

Kin selection. Scientists speculate that altruism may be maintained if the genes producing it help a genetic relative and hence give an advantage to those altruistic genes. The same could be true of homosexuality. Insofar as homosexuals have been freed from investing time and energy in their own reproduction, perhaps they are able to help their relatives rear offspring, to the ultimate evolutionary benefit of any homosexuality-promoting genes present in those children.

Unfortunately, available evidence does not show that homosexuals spend an especially large amount of time helping their relatives, or even interacting with them. Not so fast, however: Those results are based on surveys; they reveal opinions and attitudes rather than actual behavior. Moreover, they involve modern industrialized societies, which presumably are not especially representative of humanity’s ancestral situations.

Some recent research has focused on male homosexuals among a more traditional population on Samoa. Known as fa’afafine, these men do not reproduce, are fully accepted into Samoan society in general and into their kin-based families in particular, and lavish attention upon their nieces and nephews—with whom they share, on average, 25 percent of their genes.

Social prestige. Since there is some anthropological evidence that in preindustrial societies homosexual men are more than randomly likely to become priests or shamans, perhaps the additional social prestige conveyed to their heterosexual relatives might give a reproductive boost to those relatives, and thereby to any shared genes carrying a predisposition toward homosexuality. An appealing idea, but once again, sadly lacking in empirical support.

Group selection. Although the great majority of biologists maintain that natural selection occurs at the level of individuals and their genes rather than groups, it is at least possible that human beings are an exception; that groups containing homosexuals might have done better than groups composed entirely of straights. It has recently been argued, most cogently by the anthropologist Sarah B. Hrdy, that for much of human evolutionary history, child-rearing was not the province of parents (especially mothers) alone. Rather, our ancestors engaged in a great deal of “allomothering,” whereby nonparents—other genetic relatives in particular—pitched in. It makes sense that such a system would have been derived by Homo sapiens, of all primate species the one whose infants are born the most helpless and require the largest investment of effort. If sufficient numbers of those assistants had been gay, their groups may have benefited disproportionately.

Alternatively, if some human ancestors with a same-sex preference reproduced less (or even not at all), that, in itself, could have freed up resources for their straight relatives, without necessarily requiring that the former were especially collaborative. Other group-level models have also been proposed, focusing on social interaction rather than resource exploitation: Homosexuality might correlate with greater sociality and social cooperation; similarly, it might deter violent competition for females.

Balanced polymorphisms. Perhaps a genetic predisposition for homosexuality, even if a fitness liability, somehow conveys a compensating benefit when combined with one or more other genes, as with the famous case of sickle-cell disease, in which the gene causing the disease also helped prevent malaria in regions where it was epidemic. Although no precise candidate genes have been identified for homosexuality, the possibility cannot be excluded.

Sexually antagonistic selection. What if one or more genes that predispose toward homosexuality (and with it, reduced reproductive output) in one sex actually work in the opposite manner in the other sex? I prefer the phrase “sexually complementary selection”: A fitness detriment when genes exist in one sex—say, gay males—could be more than compensated for by a fitness enhancement when they exist in another sex.

One study has found that female relatives of gay men have more children than do those of straight men. This suggests that genes for homosexuality, although disadvantageous for gay men and their male relatives, could have a reproductive benefit among straight women.

To my knowledge, however, there is as yet no evidence for a reciprocal influence, whereby the male relatives of female homosexuals have a higher reproductive fitness than do male relatives of heterosexual women. And perhaps there never will be, given the accumulating evidence that female homosexuality and male homosexuality may be genetically underwritten in different ways.

A nonadaptive byproduct. Homosexual behavior might be neither adaptive nor maladaptive, but simply nonadaptive. That is, it might not have been selected for but persists instead as a byproduct of traits that presumably have been directly favored, such as yearning to form a pair bond, seeking emotional or physical gratification, etc. As to why such an inclination would exist at all—why human connections are perceived as pleasurable—the answer may well be that historically (and prehistorically), it has often been in the context of a continuing pair-bond that individuals were most likely to reproduce successfully.

There are lots of other hypotheses for the evolution of homosexuality, although they are not the “infinite cornucopia” that Leszek Kolakowski postulated could be argued for any given position. At this point, we know enough to know that we have a real mystery: Homosexuality does have biological roots, and the question is how the biological mechanism developed over evolutionary time.

Another question (also yet unanswered) is why should we bother to find out.

There is a chilling moment at the end of Ray Bradbury’s The Martian Chronicles, when a human family, having escaped to Mars to avoid impending nuclear war, looks eagerly into the “canals” of their new planetary home, expecting to see Martians. They do: their own reflections.

It wasn’t terribly long ago that reputable astronomers entertained the notion that there really were canals on Mars. From our current vantage, that is clearly fantasy. And yet, in important ways, we are still strangers to ourselves, often surprised when we glimpse our own images. Like Bradbury’s fictional family, we, too, could come to see humanity, reflected in all its wonderful diversity, and know ourselves at last for precisely what we are, if we simply looked hard enough.

Unlike the United States military, with its defunct “don’t ask, don’t tell” policy, many reputable investigators are therefore asking … not who is homosexual, but why are there homosexuals. We can be confident that eventually, nature will tell.