The authors consider social, genetic, evolutionary, and hormonal transfer hypotheses for same-sex romantic preferences of adolescent (N=5,552) sibling pairs drawn from a nationally representative sample. They show that male but not female opposite-sex twins disproportionately report same-sex attraction; and that the pattern of concordance of same-sex preference among siblings is inconsistent with a simple genetic influence model. Their results provide substantial support for the role of social influences, reject the hormone transfer model, reject a speculative evolutionary theory, and are consistent with a general model that allows for genetic expression of same-sex attraction under specific, highly circumscribed, social conditions.