In 1991, when I was a neuroscientist at the Salk Institute in San Diego, I published a study describing a difference in brain structure between gay and straight men. Since that time I've given many public lectures on the biology of sexual orientation. One of the commonest questions I get is this: If being gay is genetic, and gay sex doesn't produce children, why don't those genes die out?
That's a sensible question, and one to which we don't yet have a crystal-clear answer. I'll suggest some possible answers in a moment. First, though, I should emphasize that not everything in biology is genetic. In the case of homosexuality, most estimates (derived from twin studies) are that genes account for no more than about half of the total causation of this trait. The remaining causes include hormonal factors operating during fetal life -- factors that may not be under genetic control.
Another point is that, while gay sex doesn't produce children, gay people often do so. No one could be gayer than Oscar Wilde, for example, yet he fathered two children. Lesbians are even more likely than gay men to be parents. But both lesbians and gay men do have fewer children, on average, than their heterosexual peers, so 'gay genes' should indeed fare poorly over the long term.
The key to understanding how gay genes survive is the realization that such genes exist, not only in gay people themselves, but also in some of their non-gay relatives. For example, the sisters of a gay man might have inherited the same gene that he did, but because different genes are thought to promote homosexuality in men and in women, this 'gay male gene' won't make them lesbian. If the gay man helps his sisters raise more children than they would otherwise be capable of, then the decreased reproductive success of the gay man might be more than compensated by an increased reproductive success of his sisters, and it would be they, not he, who pass the gene on to the next generation. This is the so-called 'kin selection' hypothesis.
Do gay men in fact help their sisters raise enough extra children to compensate for their own lower fecundity? Although gay men are generally well-disposed towards their sisters, nephews, and nieces, most studies fail to find that they offer enough support -- financial or otherwise--to significantly increase their sisters' reproductive success. (An exception is offered by anthropologists Paul Vasey and Doug VanderLaan of Lethbridge University: they report that feminine gay men in American Samoa -- men who are locally referred to as fa'afafine -- do provide substantial child-rearing assistance to their sisters.)
Another possibility is that these sisters (and other female relatives) of gay men might have extra children, not on account of any assistance offered by the gay men, but by the direct action of 'gay male genes' within themselves. After all, if we think of gay male genes as genes that increase sexual attraction to males, then these genes might actually benefit the reproductive success of heterosexual women who possess them -- by making them even more sexually attracted to males than they otherwise would have been, and therefore more sexually active. Conversely, 'lesbian genes' might increase the reproductive success of heterosexual men by the same mechanism.
Some support for this hypothesis comes from studies conducted in Italy and in Britain, which have found that women with gay male relatives -- and who might therefore carry a gay male gene, do have significantly more children than women with no such relatives. This effect was seen even for women whose gay male relatives cropped up in a later generation, so that the men were not in a position to provide any material assistance to those women when they were having children. Still, there are also studies that fail to find this effect, so it's not yet clear whether it could be a general mechanism for the persistence of gay genes.
Yet another idea is that it's the same-sex relatives of gay men who enjoy a reproductive advantage. How would that work? According to economist Ed Miller of the University of New Orleans, several genes are floating around in the gene pool that make men more feminine in a variety of respects. Men who happen to inherit one or two of these genes are still heterosexual, Miller suggests, but their mild femininity -- which shows itself in the form of decreased aggressiveness, greater empathy, and the like -- makes them more attractive to women, and thus able to have more children, as compared with straight men who lack such genes. When more than two or three such genes happen to end up in the same man, the process of feminization increases to the point that it includes sexual attraction to males -- that is, it makes their owner gay. Such a model is certainly consistent with the observation that gay men are, on average, considerably more feminine that straight men in a variety of psychological traits.
According to a research group led by Brendan Zietsch of the Queensland Institute of Medical Research, feminine straight men do have more female sex partners than more masculine men do. Thus the positive effect of feminizing genes on the reproductive success of straight men could outweigh their negative effect on gay men. Similarly, Zietsch's group found that masculinity in straight women is associated with a larger number of male sex partners, suggesting that Miller's hypothesis could explain the persistence of lesbian genes too.
As a happy homosexual, I find it a bit disconcerting that my sexual orientation might simply be the price that evolution pays to improve straight men's performance in the sexual marketplace. Whether any of these explanations for the persistence of gay genes is correct, however, is a matter of speculation -- and it will likely remain so until these genes have been identified and their mechanism of action understood.
Simon LeVay, Ph.D., is the author of Gay, Straight, and the Reason Why: The Science of Sexual Orientation (Oxford University Press, 2011).