The strongest data we have on the cause of male homosexuality is the fraternal birth order effect, which says that the likelihood of being gay increases with each additional older brother. This phenomenon can be interpreted as a genetic effect if the hypothetical gene(s) predispose women to greater fertility - more likely to have more children. That predisposition to have more children, coupled to a predisposition to launch an immune response to cellular 'male-ness' could explain the fraternal birth order effect.

This finding was supported by a 2000 paper by Williams et al, which found that finger length ratio in gay men, a physical characteristic set before birth, is related to their fraternal birth order.