The evidence on nature, nurture, and sexual orientation just got a lot better.  (For earlier discussion, see here, here, and here).  Gene Expression summarizes the latest evidence:

Three recent twin studies have largely overcome previous
methodological issues, demonstrate clear genetic influences on sexual
orientation… These studies all used large,
population-based samples – that is, the subjects were not recruited to
the study based on sexual orientation…


Estimates of genetic influences were high across all three studies.
The Australian study found heritability of 48% for sexual orientation
across males and females together. The Finnish study estimated genetic
influences on sexual orientation of 45% and 50% for men and women,
respectively. Neither study found any evidence of an effect of shared
environment. The Swedish study gave somewhat different results – the
heritability for male heterosexuality was quite high, 39%, with no
effect of a shared environment. However, the estimated heritability for
female heterosexuality was lower in this study, around 18-19%, and a
significant contribution from the shared environment was found for
females in this study (16-17%).

I’m a little worried by the studies’ high estimates of the prevalence of homosexuality; it suggests that gays were more likely to respond.*  And if gays were more likely to respond, then gay pairs of twins are especially overrepresented.  By itself, however, such a response bias would actually depress estimates of genetic effects and inflate estimates of shared environmental effects. 

Not clear why?  Suppose that people only responded if both were gay.  Then the concordance difference between identical and fraternal twins would be zero, indicating no genetic effect, even though siblings’ concordance rate would be far above the population average, indicating a massive nurture effect.

* I haven’t read the original studies yet, so I apologize if any of the researchers somehow avoided this problem.