The more the mother gave birth to a boy, the more likely that younger brothers will be gays. With the sisters this pattern does not appear. The reason, according to scientists, lies in the interaction of the immune system of the mother and fetus. Full of confidence, however, it does not have.
Previously it was thought that the reason of homosexualism is psychological: a boy growing in a family with older brothers and sexual games with them left on the younger brothers a larger footprint. But the effect is seen even when the older brother died young before birth.
The study was conducted in the Psychiatric Institute Clark by the psychologist Ray Blanchard, who interviewed previously selected group of gay and straight people to 604 people.