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Genetic overlap of same-sex attraction and mating success

Genetic factors that influence same-sex attractions may also convey reproductive advantage in women and so could explain how homosexuality passes down the generations, according to a new study led by the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King’s College London, published today in the Journal of Sexual Medicine.

Researchers found the genes that make some women non-heterosexual might be the same genes that increase heterosexual women’s mating success. These common genes enhance their ‘gender nonconformity’, which makes them more psychologically masculine and is likely to lead to more sexual partners through their lifetime, said the study that came from a collaboration between the IoPPN, the Faculty of Life Sciences & Medicine also at King’s and the University of Zurich.

The characteristics of sexual orientation, gender nonconformity, and number of sexual partners were studied and each were found to be partially determined by an individual’s genes. Genes were found to account for 30% of variation in sexual orientation, for 32% of variation in gender nonconformity, and for 50% of mating success, measured by lifetime sexual partners. Importantly, the genes responsible for these three characteristics overlap.

“It appears that there is a genetic overlap between sexual orientation, gender nonconformity, and number of sexual partners,” said Dr Qazi Rahman, Senior Lecturer at the IoPPN at King’s who led the research. “We found more masculine women tended to be non-heterosexual, but when they were heterosexual they reported more lifetime sexual partners.

“This could provide a way for a trait like homosexuality to be passed on from generation to generation. The genes that make someone gay in one individual, might confer reproductive advantages to their relatives and thus homosexuality is passed on and survives evolutionary pressures.”

The study asked 996 British female twins anonymous questions about sexual attraction towards men and women to measure sexual orientation, about childhood gendered interests to measure gender nonconformity, and about their number of sexual partners through their lifetime. The more open definition of ‘non-heterosexuality’, rather than homosexuality, was used because it includes any same-sex attraction, and fits better with the observation than women show greater sexual fluidity.

The common genes that underpin the three characteristics studied were named by researchers as an individual’s level of ‘sex typicality’. Flexibility and diversity in an individual’s levels of sex-typicality has advantages when seen from an evolutionary perspective, but although these genes are heritable, environmental factors also have a large part to play in sexual preferences and behaviour.

Professor Tim Spector, King’s College London, said: “Twins are the perfect model to sort out the effects of nature and nurture even for complex traits like homosexuality. Female homosexuality is clearly a mix of genes and environment. These results offer an explanation as to why these genes have persisted across generations and not died out. Women with these genes are slightly more promiscuous than average. In the past this would have led to greater fertility in their relatives, counterbalancing their non-fertile same sex partnerships.”

Dr Rahman concluded: “We now need to pay attention to how biology and culture interact to generate the diversity of sexuality and gender expression in humans. The cultures we live in and the environmental factors that contribute to sexual behaviour and gender identity will also need close study.”

Paper reference: Rahman, Q. et al. ‘Common Genetic Factors among Sexual Orientation, Gender Nonconformity, and Number of Sex Partners in Female Twins: Implications for the Evolution of Homosexuality’ published in the Journal of Sexual Medicine

For a copy of the paper or interview with the author, please contact Tom Bragg, Press Officer, Institute of  Psychiatry, Psychology & Neuroscience, King’s College London / (+44) 020 7848 5377 / (+44) 07887 793 556