Before leaving twin studies behind, I just wanted to highlight a couple of other interesting findings from the twin studies that relate to the conventional wisdom about homosexuality. These are from the Australian Twin study published in Journal of Personality and Social Psychology, 2000, Volume 78, pages 524-536.
Distribution of Sexual Orientation
This study was performed in the twin registry and the results are expected to largely mirror the general population. The study investigators looked at the distribution of Kinsey scores in their population. ~92% of both men and women in the sample had Kinsey scores of 0, giving an approximate 8% prevalence of non-heterosexuality. (As an aside, the Swedish twin study found a 5.6% prevalence for men who had ever had a same-sex partner and 7.8% prevalence for women who had ever had a same-sex partner, and an American study found a 3% prevalence of non-heterosexual orientation). Interestingly the distribution between the other scores differed between men and women. Women were more likely to have a Kinsey score of 1 with decreasing likelihood for subsequently higher Kinsey scores. Men, on the other hand, were increasingly likely to have higher (5 and 6) Kinsey scores, supporting the observation that men are less likely to be bisexual.
Homosexual Sexual Activity
The study also found that individuals with Kinsey scores of "0" were unlikely to have homosexual sexual experiences (although nearly 10% of such men reported having had a same-sex experience), but 60% of men with a Kinsey score of "1" were likely to have had at least one same-sex partner, 70% of men with Kinsey scores of "2-4" were likely to have had at least one same-sex partner, and nearly 100% of men with Kinsey scores of "5-6" were likely to have had at least one same-sex partner. Unfortunately the study did not show the likelihood of different Kinsey scores to have opposite-sex partners (which I'd be interested in, as I am in a MOM).
Childhood Gender Nonconformity
The study also looked at Kinsey scores and reported childhood gender nonconformity, such as the likelihood to have participated in sex-stereotypic games and activities. The study found an increasing likelihood to have have not conformed to childhood gender stereotypes with increasing Kinsey score. Perhaps this helps explain some of my childhood experiences.