Homosexuality, the natural choice? (Gay Animals are Common)

But in any case, homosexuality doesn't have to be adaptive in a strict genetic sense to be a real phenomenon, argues Bagemihl. "Researchers have been blinded by the prevailing preoccupation to find adaptive explanations for every behaviour," agrees Vasey. He spent his doctoral years hunting in vain for evidence to support such explanations for the sexual proclivities of female Japanese macaques. For instance, the females do not use sex to test or establish dominance ranks or to form social bonds; they form a liaison, and when it is over, they act as though it never happened.

Much of my early work on sexual orientation focused on behavioral genetics. I did several twin and family studies, which suggested that both male and female homosexuality run in families, and that male and female sexual orientation are moderately (but far from completely) heritable. You can download the most recent twin study.

Because identical (monozygotic, or MZ) twins are often discordant for homosexuality, environment must matter. It is important to realize, though, that "environmental" is not equivalent to "social." There can be biological causes of MZ twin differences. We hope to begin a study of discordant MZ twins (i.e., twin pairs with one homosexual and one heterosexual twin).With collaborators Alan Sanders, Khytam Dawood, Elliot Gershon, and others, we have begun a genetic linkage study to try to replicate Dean Hamer's famous finding of linkage on chromosomal site Xq28, and to search for other linkage sites. This study will take several years, but we expect a definitive answer to the question whether there is linkage at Xq28.