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
| 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
| 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.