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.
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."
Will Saletan covers science, technology, and politics for Slate and says a lot of things that get him in trouble.
Illustration by Mark Alan Stamaty. Photograph of a gay couple on Slate's home page by Medioimages/Photodisc/Getty Images.