Working mothers are probably more likely to have a degree and households with two educated parents typically lead to more successful children.

Stay at home moms probably also are more likely to raise girls that become stay at home mothers. That would bring average salaries of their children down.

Also working mothers are more likely to live in a city which means their children do too. Their children would then be more likely to have a higher salary.

I’d like to see a study that accounts for all of the related factors.