We present an overview of behavioral genetics research on homosexual and heterosexual orientation. Family, twin, and adoptee studies indicate that homosexuality and thus heterosexuality run in families. Sibling, twin, and adoptee concordance rates are compatible with the hypothesis that genes account for at least half of the variance in sexual orientation. We note observations of homosexual behavior in animal species, but the analogy to human sexual orientation is unclear. We discuss the reproductive disadvantage of a homosexual orientation and present possible mechanisms that could maintain a balanced polymorphism in human populations.