Do studies of identical twins show that there is a relationship between genetics and sexuality?

Answer 1

Yes. Absolutely.

In the Kallman study in 1952, Kallman found that of the 37 identical twins classified they all not only had the same sexuality but were similar in their sex practices.

The next large study of note (and the most recent) was performed in 1991 by Bailey and Pillard. They decided to look for homosexuals to apply to a study and asked them if they had an identical twin, fraternal twin, or an adoptive brother. Then they asked for the sexuality of the brothers of the homosexual men. The scientists believe that if genetics is a factor, we should see a much higher amount of homosexual behavior in identical twins than in fraternal twins who only share as many genes as normal siblings. Also, if genetics play a part in sexuality, then there should still be a higher concurrence in fraternal twins than in adoptive brothers who share no genes.

52% (more than half) of the identical twins of a homosexuals were also homosexual. As the average amount of people that are homosexual is around 10%, this indicates that genetics play a very large role in sexuality.

22% of the fraternal twins of homosexuals were also homosexual. Again, this is in comparison to the 10% of the population that is normally homosexual, so even with brothers that are not identical genetics make it twice as likely that a person will be homosexual if they have a homosexual sibling.

Finally, with adoptive brothers that shared the same home, only 11% were homosexual. When no genetics are involved, the rate of homosexuality dropped down to where it would be expected to be - the national average of 10%.