Sexual Orientation in a U.S. National Sample of Twin and Nontwin Sibling Pairs

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Abstract
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 mon...