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Sexual Antagonism

A genetic theory of homosexuality

William Saletan, Slate, June 25, 2008

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."  
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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