The most unacknowledged spending expectation among women is the amount of time spent by single mothers caring for children, not only physically, but psychologically. It is my feeling that only a small percentage of a mother's time is normally compensated for by child support, given what a woman could make adding these hours to workforce hours… It is why women who have never been married and never had children earn so much more in the workplace than women who have had children.

p. 222. - Why Men Earn More (2005)