Blanchard has conducted research on factors that influence the development of sexual orientation, including biological factors. He has proposed a theory known as a fraternal birth order effect or "older brother effect". This theory is that the more older brothers a man has, the greater the probability is that he will have a homosexual sexual orientation. The number of older sisters has no effect, however. The same is not true for lesbians—neither the number of older brothers nor the number of older sisters appears to be related to the sexual orientation of women. The fraternal birth order effect is the strongest known predictor of sexual orientation, with each older brother increasing a man's odds of being gay by about 33%. Blanchard goes on to state that "A man's chance of being gay is pretty low to begin with --perhaps as low as 2%..." So having one older brother ups the chance to only about 2.6%."
Blanchard hypothesizes that the older brother effect is caused by interactions between a male fetus and the immune system of the mother: Because certain proteins (called H-y antigens) are produced by male and not by female fetuses, the mother's immune system reacts only to male fetuses and is more likely to produce a reaction with each successive exposure to a male fetus.
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