Who you have sex with....

Genetic and Environmental Effects on Same-sex Sexual Behavior: A Population Study of Twins in Sweden:

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.

ScienceDaily has an important caveat:

...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.

In plain language these results suggest that the largest proportion of the variance within the population for this trait (same sex orientation) can not be accounted for by variation in genes or shared family outlook/experiences. They also shouldn't be surprising, Judith Rich Harris has written two books, The Nurture Assumption and No Two Alike, attempting to explain the "non-shared environmental" aspect of variance which pops up in behavior genetic studies (the breakdown is often on the order of 50% genetic, 10% shared environment, and 40% non-shared environment). Do note that that component of variance may still have biological underpinnings; e.g., an environmental shock during the prenatal stage which changes the path of development. Finally, I do think it is intriguing that females show a much larger contribution of shared environment. To me this dovetails with the anecdotal observation that facultative homosexual behavior is more common among females than males, especially when you exclude extreme situations such as imprisonment or living in Saudi Arabia.

Also, if you're interested, Gay Men, Straight Women Have Similar Brains. The study will be published in PNAS when they get around to it....

Tags
Categories

More like this

The New Jersey Supreme Court just came down with a ruling in a gay marriage case that is similar to what Vermont did a few years ago.
At least to me, this poll in the Baltimore Sun looks like a no-brainer. Which would imply that 59% of the people in Maryland have no brains. Think of the zombies, and do something to correct this problem! Recognizing same-sex marriage
This morning, the California Supreme Court will announce its decision on whether or not a slim 50% majority can amend the state constitution in order to specifically deny a previously-recognized constitutional right.
"Stanley Kurtz is the 'EverReady Bunny' of the same-sex marriage debate, a character who moves forward unrelentingly on a quest to prove that same-sex marriages are harmful. He is sure that state recognition of lesbian and gay unions in Europe has harmed the institution of marriage.

Finally, I do think it is intriguing that females show a much larger contribution of shared environment. To me this dovetails with the anecdotal observation that facultative homosexual behavior is more common among females than males

The thing to remember is that female sexuality is more environmentally shaped in general than male sexuality.

Related: Lesbians have experienced more abusive heterosexual relationships.

By Jason Malloy (not verified) on 17 Jun 2008 #permalink

That quote from ScienceDaily isn't quite correct. In a twin study, pregnancy is a shared environment. In particular, this study seems like a blow to the pre-natal infection theory.

By Douglas Knight (not verified) on 17 Jun 2008 #permalink