Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample.

TLDR

We recruited twins from the Australian Twin Registry and assessed their sexual orientation along with childhood gender nonconformity and continuous gender identity. Men and women differed in sexual orientation distributions, with women showing slight‑to‑moderate homosexual attraction and men high degrees, twin concordances were lower than prior studies, familial factors were important but could not separate genetic from shared environmental influences, except that childhood gender nonconformity was heritable in both sexes, and multivariate analyses indicated a sex‑specific causal architecture with genetics playing a significant role for women.

Abstract

We recruited twins systematically from the Australian Twin Registry and assessed their sexual orientation and 2 related traits: childhood gender nonconformity and continuous gender identity. Men and women differed in their distributions of sexual orientation, with women more likely to have slight-to-moderate degrees of homosexual attraction, and men more likely to have high degrees of homosexual attraction. Twin concordances for nonheterosexual orientation were lower than in prior studies. Univariate analyses showed that familial factors were important for all traits, but were less successful in distinguishing genetic from shared environmental influences. Only childhood gender nonconformity was significantly heritable for both men and women. Multivariate analyses suggested that the causal architecture differed between men and women, and, for women, provided significant evidence for the importance of genetic factors to the traits' covariation.