A mother's immune system may have a defensive response to substances produced by male fetuses. This is most likely to contribute to

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The fraternal birth-order effect is a phenomenon in which the more older brothers a male has, the higher the likelihood that he will be homosexual in adulthood. One theory for this effect is that a mother's immune system may develop a defensive response to substances produced by male fetuses, which may then affect the development of subsequent male fetuses in a way that increases the likelihood of homosexuality.

A Mother's Immune System May Have a Defensive Response to Substances