It has been suggested that the older-brother effect is a result of

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The older-brother effect is a phenomenon where a man's chance of being homosexual increases with the number of older brothers he has. It has been suggested that this is due to a maternal immune response, where a mother's body develops an immune response to male-specific antigens during pregnancy, which can affect the development of the brain in subsequently born male fetuses. Gender typing, refractory periods, and mass-media norms are not related to this effect.

It Has Been Suggested That the Older-Brother Effect Is a Result