While some scientists search frantically for "gay" genes, a University of Toronto researcher has discovered a hitherto unsuspected contributor to male sexual orientation: having lots of older brothers.
According to psychologist Ray Blanchard, each older brother that a boy has raises his chances of being gay by a third over the rate in first-born sons. "It might go from a 3 percent probability of homosexuality with the first son to 4 percent for the second to 5 percent for the third," explains Blanchard.
There may be psychosocial reasons why a brother glut would increase the odds of being gay, but Blanchard thinks the immune system is to blame. Pregnant moms sometimes have immune reactions against male-only proteins on a boy fetus. If immune cells "remember" earlier pregnancies and react more strongly with each one, Blanchard posits, then they might attack and alter fetal brains cells that will later influence sexual behavior.
from Psychology Today
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