A scene from the Copenhagen Pride Parade.

Enlarge / A scene from the Copenhagen Pride Parade. (credit: Ole Jensen/Getty Images)

Homosexuality is widespread among animals, and it's present in every culture we've looked at. All of which suggests that it's in some way innate to the human species. But figuring out what "innate" means is a real challenge when it comes to a complicated suite of behaviors like sexuality—behaviors that are also heavily influenced by culture.

Studies of twins have suggested that genetics can influence homosexual behavior, accounting for roughly 20% to 30% of its frequency. But attempts to find specific genes that mediate this influence have come up empty. Now, an absolutely enormous study has found a number of genetic sites linked to homosexual behavior. But collectively, they account for a tiny amount of the total genetic effect, and their influence is complicated: different in men and women, and different even across the spectrum of sexual attraction.

Very big data

There are a number of human traits that have a clear genetic influence but aren't clearly tied to any single gene. Height is a good example. Height is influenced by a large number of genes, each of which on its own has only a tiny effect—there's no "tall gene." And, obviously, environmental influences like nutrition are also strong influences on height and can completely swamp the impact of genetics.

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