Genetic and Environmental Effects on Same-sex Sexual Behavior: A Population Study of Twins in Sweden
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There is still uncertainty about the relative importance of genes and environments on human sexual orientation. One reason is that previous studies employed self-selected, opportunistic, or small population-based samples. We used data from a truly population-based 2005–2006 survey of all adult twins (20–47 years) in Sweden to conduct the largest twin study of same-sex sexual behavior attempted so far. We performed biometric modeling with data on any and total number of lifetime same-sex sexual partners, respectively. The analyses were conducted separately by sex. Twin resemblance was moderate for the 3,826 studied monozygotic and dizygotic same-sex twin pairs. Biometric modeling revealed that, in men, genetic effects explained .34–.39 of the variance, the shared environment .00, and the individual-specific environment .61–.66 of the variance. Corresponding estimates among women were .18–.19 for genetic factors, .16–.17 for shared environmental, and 64–.66 for unique environmental factors. Although wide confidence intervals suggest cautious interpretation, the results are consistent with moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment (social and biological) on same-sex sexual behavior.
KeywordsSexual behavior Sexual orientation Population survey Twin study
We are grateful to Dr. Ray Blanchard who generously provided validation data from men referred to the Kurt Freund Laboratory at the Centre for Addiction and Mental Health in Toronto, Ontario. The Swedish Twin Registry is supported by unrestricted grants from the Swedish Department of Higher Education, the Swedish Research Council, and AstraZeneca. Niklas Långström is supported by the Swedish Research Council-Medicine.
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