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Stockholm, Sweden. The attitudes of families and the public exert little impact on adult decisions to have sex with persons of the same or the opposite sex. Instead, findings from the largest study in the world so far point to hereditary factors and individual experiences as the strongest influences on our choice of sexual partners.
The conclusions apply equally well to why people only have sex with persons of the opposite sex as to why we have sex with same-sex partners. However, the conclusions are more difficult to transfer to countries where non-heterosexual behaviour remains prohibited.
The study from the Swedish medical university Karolinska Institutet (KI) was performed in collaboration with the Queen Mary University of London. More than 7,600 Swedish twins (men and women) aged 20-47 years responded to a 2005 - 2006 survey of health, behaviour, and sexuality. Seven percent of the twins had a same-sex sexual partner.
"The results show, that familial and public attitudes might be less important for our sexual behaviour than previously suggested", says Associate Professor Niklas Långström, one of the involved researchers.
"Instead, genetic factors and the individual's unique biological and social environments play the biggest role. Studies like this are needed to improve our basic understanding of sexuality and to inform the public debate."
Overall, the environment shared by twins (including familial and societal attitudes) explained
0-17% of the choice of sexual partner,
genetic factors 18-39% and
the unique environment 61-66%.
The individual's unique environment includes, for example, circumstances during pregnancy and childbirth, physical and psychological trauma (e.g., accidents, violence, and disease), peer groups, and sexual experiences.
Genetic and Environmental Effects on Same-sex Sexual Behavior: A Population Study of Twins in Sweden. Niklas Långström, Qazi Rahman, Eva Carlström, Paul Lichtenstein. Archives of Sexual Behavior 7 Jun 2008. doi: 10.1007/s10508-008-9386-1. ISSN 0004-0002 (Print) 1573-2800 (Online).
Abstract
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.
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