The latest numbers estimate that as many as 20 percent of humans are homosexual, and given that the majority of scientists think that homosexuality is a trait present from birth, those sorts of numbers indicate that whatever part of homosexuality is classically heritable can’t have been too strongly selected against. In highly social species like ours, the benefits of having extra hands for child rearing—hands that aren’t busy taking care of genetic children of their own—might have outweighed the evolutionary pressure against homosexuality. Homosexuality has been observed in countless species,
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