Sexual orientation in a U.S. national sample of twin and nontwin sibling pairs.

OBJECTIVE Although previous studies have suggested that sexual orientation is influenced by familial factors, which may be partly genetic, these studies have relied on unrepresentative and potentially biased samples. The authors attempted to estimate the role of genetic and environmental factors in the determination of sexual orientation in a more representative sample. METHOD Sexual orientation was assessed by a single item on a self-report questionnaire in a U.S. national sample of twin and nontwin sibling pairs. Sexual orientation was classified as heterosexual or nonheterosexual (bisexual or homosexual). The authors compared the similarity of sexual orientation in the monozygotic twins to the similarity in the same-sex dizygotic twins, all dizygotic twins, the same-sex dizygotic twins and sibling pairs, and all dizygotic twins and sibling pairs. Biometrical twin analyses were performed. RESULTS All analyses demonstrated familial resemblance for sexual orientation. Resemblance was greater in the monozygotic twins than in the dizygotic twins or in the dizygotic twins plus nontwin siblings. Biometrical twin modeling suggested that sexual orientation was substantially influenced by genetic factors, but family environment may also play a role. No evidence was found for a violation of the equal-environment assumption regarding monozygotic and dizygotic twin pairs. CONCLUSIONS Familial factors, which are at least partly genetic, influence sexual orientation. The results of these analyses should be interpreted in the context of low statistical power and the use of a single item to assess the complex phenotype of sexual orientation.