Variations in male sexual orientation: The role of maternal responsivity to male-specific proteins

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The fraternal birth order (FBO) effect—that, on average, gay men have a higher number of older brothers than heterosexual men—is a reliable correlate of men's sexual orientation. This effect is likely to arise before birth, with the most plausible explanation being that during pregnancy a mother develops an immune response to a substance important in male fetal development, an effect that becomes more likely with each male gestation and is thought to cause an alteration in the brain development of some later born males. The target of the response may be protein molecules on the surface of fetal brain cells so that maternal antibodies binding to them alter their role in typical sexual differentiation, leading some later born males to be attracted to men not women.