Interesting

Having older brothers a factor in boys becoming gay

It's not a social force, either, but rather the very fact of the mother carrying boys in previous pregnancies that may change the womb environment to produce a child who is going to be gay. There is thought that the woman's body reacts to the XY chromosomes as more foreign than if she were carrying daughters, and so for each brother a boy's chances of being gay go up by about a third, according to the study. Having sisters has no effect, and likewise does not effect girls who become lesbian. In lesbians, there seems to have been a higher amount of testosterone than normal in the womb. Both of these findings support homosexuality as a result of genetics and prenatal effects rather than as a lifestyle choice.