Scienctific American, By Jesse Bering
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Although the precise genetic mechanisms underlying homosexuality are still relatively unknown, we do know that, however these mechanisms actually work, there are indeed clear, contributing genetic factors underlying homosexual orientation.
The best evidence that homosexuality runs in families as a heritable biological trait comes from 1990s-era twin studies, which revealed that the concordance rate (the rate by which twin members overlap on anything from schizophrenia to creativity to sexual orientation) for homosexuality is significantly greater in monozygotic twins (identical) than in dizygotic twins (who share only half of their genes, just like non-twin siblings).
The more rigorously controlled twin studies adjust for possible shared environmental influences by taking into account, for instance, the sexual orientation of non-twin siblings or twins separated at birth, and yet all reveal that homosexuality is at least partially heritable.
Homosexuality is often presented as an evolutionary “mystery” because of the obvious reproductive disadvantages, and thus for decades researchers have sought some adaptive function for the culturally recurrent percentage (anywhere from 1 to 10 percent of the population, depending on the measures used) of the human population that is aroused more by the same than it is by the opposite sex. Yet if we consider the historical, and perhaps even the ancestral, percentage of the homosexual population that did in fact reproduce because of societal proscriptions against adult relations with the same sex, the mystery becomes considerably less profound.
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