The 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 refers to the phenomenon where there is an increased likelihood of men being homosexual with each older brother they have. One theory for this effect is that the mother's immune system may develop a defensive response to substances produced by male fetuses, which can affect brain development in subsequent male fetuses.

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