Friday, December 16, 2005

Sexuality and Genes

Conrad, a commenter on a recent post about Brokeback Mountain and whether aversion to witnessing homosexual affection is genetic, opines that:

if one accepts that sexual orientation is genetic, that homosexuality is in the genes, it makes tremendous sense to assert that hetero aversion to gay sex is also in the genes. The point being that it's very hard to imagine a genetic reason why homosexuality would pass on homosexual genes to the next generation, since gay sex doesn't produce children. Whereas it makes excellent sense to suggest that hetero men with an aversion to gay sex are more likely to pass on their genes than hetero men without an aversion to gay sex.
Conrad makes some reasonable points, but I don’t necessarily agree. First, let me address the point that “it’s very hard to imagine a genetic reason why homosexuality would pass on homosexual genes to the next generation.” Then I will address whether hetero aversion to gay sex is likely to be genetic, because it enhances genetic fitness.

It is far from certain that homosexuality has a genetic component, but if it does, there are several theories of how genes that promote homosexuality could be maintained in a population. An early theory, by sociobiology founding father E.O. Wilson held that homosexuality is a form of genetic altruism—that homosexuals through human pre-history refrained from parenthood and instead devoted themselves to being good aunts and uncles—to helping raise the children of the tribe with whom they shared some genes. This theory hasn’t held up very well, because it’s hard to imagine aunt/uncle effort contributing sufficiently to genetic success to explain the persistence of genes that discouraged parenthood itself.

Another theory, first explained by Richard Dawkins and more recently by Robert Wright, holds that there may be genes that in modern society predispose some individuals to homosexuality, but that these genes did not predispose people to homosexuality in environments during which humans did most of their evolving. The expression of any gene is dependent on its environment, and maybe hunter-gatherer societies were not conducive to development of homosexuality even in people who, if alive today, would be homosexual. Keep in mind that in hunter gatherer societies there were no bars, personals ads, clubs, literature, movies or television and that people lived in very small groups and knew only a small number of people. Even for people who had some tendency towards homosexuality in a hunter-gatherer society, it might be difficult to find or identify other homosexuals as sexual partners or role models. Unable to express their homosexual tendencies, according to this theory, they “became” heterosexual and passed on their genes.

Conrad draws an analogy between sibling incest aversion and aversion to homosexuality. But incest aversion is a different case, because a very high percentage of people throughout human evolution have opposite sex siblings that are theoretically available as sexual partners, and because incest has hugely negative fitness implications, especially for women but for men too. So genes that predispose a person to finding sex with a sibling to be disgusting have high fitness benefits.

Another theory is that there may be genes that promote homosexuality but that have other fitness benefits. Perhaps a number of genes, each of which individually has fitness benefits and do not promote homosexuality, when combined together predispose a person to be homosexual. Still another theory, applicable only to male homosexuality, is that there may be genes that promote male homosexuality on the X chromosome, and that these genes have a fitness benefit to women. Since women have two X chromosomes and men only one, an X chromosome gene as it is passed through the generations will spend 2/3 of its time in women’s bodies, and only 1/3 in men’s. Therefore, if there were an X chromosome gene that, say, decreased a woman’s odds of dying in childbirth while predisposing a man to homosexuality, such a gene might survive in a population.

I personally think that some combination of the Dawkins theory of different environments and the gene combination/X chromosome theory is plausible, but no one really knows.

Turning now to the question of whether aversion to gay sex is genetic, I find it highly plausible that most men are genetically predisposed to prefer women as sexual partners, and vice versa. It’s less clear to me, however, that Conrad is right to assert that “hetero men with an aversion to gay sex are more likely to pass on their genes than hetero men without an aversion to gay sex,” if by this he means aversion to other people having gay sex. An aversion to the type of sex that other people are having doesn’t obviously seem to be fitness-enhancing. Maybe the point is that an aversion to gay sex makes a man more hetero on a continuum from homosexuality to heterosexuality, more exclusively devoted to sex with females and therefore less likely to expend effort on genetically unproductive couplings. Perhaps. Or perhaps aversion to other people having gay sex is a byproduct of genes that make a person prefer not to have gay sex themselves.

Though such theories have some plausibility, I’m reluctant to accept them because it is so easy to spin out equally plausible theories of what ought to be in our genes. Why not theorize that men should be genetically predisposed to encourage other men to be homosexual in order to keep the women for themselves, or that people ought to be averse to masturbation and oral sex because they’re wasted effort genetically.

Moreover, human beings have all sorts of genetic predispositions that may or may not be expressed depending on the circumstances. Perhaps a predisposition to disgust at homosexual affection between others is expressed only in when an individual rarely witnesses homosexual expressions of affection. So to say that heterosexual men may have a genetic predisposition to find expressions of homosexuality in others to be disgusting, seems to me to be a way to say, “it will ever be so.” I’m skeptical. If you said that genetic programming is likely to ensure that even 1000 years from now, most people will prefer opposite sex sexual partners and that most people will continue to feel protective towards their children and to prefer comfort over misery, I would agree. The genetic benefits of these tendencies, across an enormous varieties of possible environments, are just so obvious.

But if you say, “heterosexual men will always find the sight of other men kissing to be disgusting, because it’s in the genes” it seems to me that you’re trying to justify preferences that are highly malleable, to limit the scope of our culture to make change. I won’t go there.