There is still uncertainty about the relative importance of genes and environments on human sexual orientation. One reason is that previous studies employed selfselected, 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.
Reactions are mixed but not really along any ideological grounds that I can see. For instance, from ScienceNOW:
J. Michael Bailey, a psychologist at Northwestern University in Evanston, Illinois, who led earlier twin studies of sexual orientation, calls the new study “good, important, and one unlikely to be bettered in the near future.” But Jonathan Beckwith, a geneticist at Harvard Medical School in Boston, says that the new work fails to overcome a number of problems faced by previous twin studies. He notes that the final sample included only 12% of the males in the Swedish registry, leaving open the possibility of recruitment bias. And Beckwith says that the failure to control for family environment could inflate estimates of genetic influence.
Co-author Qazi Rahman, was quoted by the Washington Post:
“This study puts cold water on any concerns that we are looking for a single ‘gay gene’ or a single environmental variable which could be used to ‘select out’ homosexuality — the factors which influence sexual orientation are complex. And we are not simply talking about homosexuality here — heterosexual behavior is also influenced by a mixture of genetic and environmental factors,” study co-author Dr. Qazi Rahman, a leading scientist on human sexual orientation, said in a prepared statement.
I intend to devote at least one more post to this study as I agree with Michael Bailey that it is an important study. I think along with the other 2 population based studies (Bailey’s in 2000 and Kendler’s also in 2000), it provides a picture of modest genetic effects along with a major role for non-shared enviromental factors. Many roads lead to a similar result. Nothing in this study provides a clear picture of what those environmental factors are but a simple environmental explanation (e.g., poor parenting) or genetic source (single gene, or uniform action of several genes) is not supported here.
Rahman added in the Post article:
“Overall, genetics accounted for around 35 percent of the differences between men in homosexual behavior and other individual-specific environmental factors (that is, not societal attitudes, family or parenting which are shared be twins) accounted for around 64 percent. In other words, men become gay or straight because of different developmental pathways, not just one pathway,” Rahman said.