Research suggests that the fraternal birth-order effect may be a result of

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The fraternal birth-order effect refers to the observation that each older brother increases the likelihood that a male child will be gay. Research suggests that this effect may be due to maternal immune responses to a protein found in male brains that are produced by some women during pregnancy. As a woman carries more male fetuses, her body may become more adept at recognizing and attacking this protein, potentially affecting the development of the male brain in subsequent pregnancies.

Research Suggests That the Fraternal Birth-Order Effect May Be a Result