Sociobiologists have proposed evolutionary explanations of homosexuality. Such hypotheses assume that the homosexual orientation is a distinct, reifiable trait, rather than an expression of universal sexual and emotional drives. For homosexuality to constitute an evolved trait, it must have a genetic basis. However, there is no reliable evidence that homosexual and heterosexual orientations are caused by genetic differences. On these and other grounds, we find sociobiological explanations of homosexuality to be implausible and unsupported by evidence. Evolutionary theory provides no guide to morality or ethical progress, nor for appropriate social attitudes toward homosexuality.