Interesting question, DiveDiveDive.
Two possibilities: 1) Mothers tend to do most of the child-raising, and most children have their primary relationship with their mother, so those N mothers who have children will have them in their control for years and have plenty of time to do damage. Plus women are expected by society to be good mothers, so N women may have a need to play the role of good mother even if they're lousy at it -- a situation which would multiply the possibilities for screwing up the kids, because they would be expected to support their mother's image as a wonderful mother when their experience is just the opposite.
2) I think a lot of men who are Ns tend to get their N supply from their public role -- like running a business or being in some other profession where they can have lots of people sucking up to them. They might not spend much time at home because they don't get the adoration they want from their wives and children -- thus the children suffer from their absence, but not as much from their N characteristics. (And men aren't automatically expected to be good fathers, so N men don't necessarily lose face with their public by being absent.)
Don't know if there's been any research on this, but it makes sense to me.
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