Another Chemical Factor In Some Homosexuals

"One of the most consistent environmental explanations for homosexuality is called the fraternal birth order effect. Essentially, the more older brothers a man has, the more likely he is to be gay. The effect doesn't hold for older or younger sisters or younger brothers, or even for adoptive or step-brothers. - - - According to Ray Blanchard, a professor of psychiatry at the University of Toronto, the reason could be that the mother’s body mounts an immune attack on the fetus of her unborn son."

How Older Brothers Influence Homosexuality