A genetic theory of homosexuality.

Science, technology, and life.
June 25 2008 8:04 AM

Sexual Antagonism

A genetic theory of homosexuality.

Gay couples can't have biological kids together. So if homosexuality is genetic, why hasn't it died out?

A study published last week in PLoS One tackles the question. It starts with four curious patterns. First, male homosexuality occurs at a low but stable frequency in a wide range of societies. Second, the female relatives of gay men produce children at a higher rate than other women do. Third, among these female relatives, those related to the gay man's mother produce children at a higher rate than do those related to his father. Fourth, among the man's male relatives, homosexuality is more common in those related to his mother than in those related to his father.

William Saletan William Saletan

Will Saletan writes about politics, science, technology, and other stuff for Slate. He’s the author of Bearing Right.

Can genes account for these patterns? To find out, the authors posit several possible mechanisms and compute their effects over time. They conclude that only one theory fits the data. The theory is called "sexually antagonistic selection." It holds that a gene can be reproductively harmful to one sex as long as it's helpful to the other. The gene for male homosexuality persists because it promotes—and is passed down through—high rates of procreation among gay men's mothers, sisters, and aunts.

This theory doesn't account for female homosexuality, which another new study (reviewed in Human Nature last week) attributes to nongenetic factors. It also doesn't account for environmental or prenatal chemical factors in male homosexuality, such as the correlation between a man's probability of homosexuality and the number of boys previously gestated in his mother's womb. But it does explain the high similarity of sexual orientation between identical twins, as well as patterns of homosexuality in families. It's also plausible because sexually antagonistic selection has been found in other species. And many scientists who think environmental and prenatal factors influence homosexuality also believe that genes play a role.

The authors note that according to their computations, the theory implies some testable predictions. One such prediction can be checked against existing data. The prediction is that on average, if you're a straight man, the reproductive pattern among your aunts will reverse the pattern seen among aunts of gay men. That is, your paternal aunts will produce children at a higher rate than your maternal aunts will. The authors check this prediction against the available data. Sure enough, it holds up.

I don't know to what extent this theory will end up explaining male homosexuality. But its emergence threatens to change our thinking about gay men in several important ways.

First, it implies natural limits to homosexuality. You don't need to worry that gay teachers or television characters will "convert" hordes of boys. Sexually antagonistic selection is self-limiting and impervious to postnatal cultural factors. The authors' computations show no scenario in which male homosexuality spreads throughout a population.

Second, by the same token, you can't culturally eradicate the gay minority. It's sustained by genetics and natural selection.

Third, if the authors are correct, we're not really talking about genes for homosexuality. We're talking about genes for "androphilia," i.e., attraction to men. The importance of the genes lies in what they do not to men but to women, by increasing reproductive output so powerfully that these women compensate for the reduced output among their male relatives. You can't isolate gay men as a puzzle or problem anymore. You have to see them as part of a bigger, stronger, enduring phenomenon.

Fourth, this larger phenomenon can't be dismissed as a disorder. The study's press release concludes that "homosexuality should not be viewed as a detrimental trait (due to the reduced male fecundity it entails), but, rather, should be considered within the wider evolutionary framework of a characteristic with gender-specific benefits."

Fifth, the benefits aren't really confined to women. They protect society as a whole. The authors' computations indicate that as a society's birthrate falls, female carriers of androphilic genes account for a larger share of the output. In short, the genes provide a "buffer effect" against extinction.

The study's lead author, Andrea Camperio Ciani of the University of Padova, sees these ramifications as a happy ending. "This is an example where the results of scientific research can have important social implications," he tells LiveScience. "You have all this antagonism against homosexuality because they say it's against nature because it doesn't lead to reproduction. We found out this is not true because homosexuality is just one of the consequences of strategies for making females more fecund."

But the word consequence suggests a sixth, less happy implication: How would gay men see themselves and be regarded in a society that understood their condition as a side effect of female evolution? Would male androphilia be treated like sickle-cell anemia—the unfortunate cost of a genetic mutation that's beneficial in other people? We medicate sickle-cell anemia. Would we medicate homosexuality?

I don't know, and neither does Dr. Camperio Ciani. Science, like culture and politics, has its happy moments. But don't mistake them for endings.

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