No-one knows why this is, but one theory is that with each male pregnancy, a woman's body forms an immune reaction to proteins that have a role in the development of the male brain.

没有人知道其中的原因，有一种理论是，每次怀上男孩，母体就会产生某种蛋白免疫反应，而这种蛋白质恰好影响男性大脑发育。

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Another theory is that the initial illness may have created an autoimmune reaction, causing the body's immune system to attack normal cells as if they were foreign substances.

CNN: Post-polio syndrome