Reports by several panelists underscored how little we know. It is important to keep in mind the fact that very few people, including twins, are exclusively homosexual. Most studies are based on 40 to 60 twin pairs at most.
Identical twins raised completely apart are rarer still.
Adoption agency criteria for adoptive families may ensure more similarities than differences. Given the small numbers and the possible similarities in environments, it is not surprising that twin studies are inconclusive. Marker Xq 28 is now officially registered as GAY-1. The LeVay study has been questioned because 19 or the 41 subjects died of AIDS, which may also affect the brain. There are many animal studies of this kind.