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.