Opposite-sex twins and adolescent same-sex attraction

The etiology of human same-sex romantic attraction is generally framed in terms of (1) social influences, (2) genetic influences, or (3) hormonal influences. In this article, we show that adolescent males who are opposite-sex twins are twice as likely as expected to report same-sex attraction; and that the pattern of concordance (similarity across pairs) of same-sex preference for sibling pairs does not suggest genetic influence independent of social context. Our data falsify the hormone transfer hypothesis by isolating a single condition that eliminates the opposite-sex twin effect we observe - the presence of an older same-sex sibling. We also consider and reject a speculative evolutionary theory that rests on observing birth-order effects on same-sex orientation. In contrast, our results support the hypothesis that less gendered socialization in early childhood and preadolescence shapes subsequent same-sex romantic preferences.