One of the reasons men make more is that they like competing more than women do. In particular, men like tournament competition, with clear winners and losers, much more than women do. In adult life, the clearest way to mark winning is with higher pay. Men are more willing to negotiate for pay; women not only don't like to negotiate, they don't like to be negotiated with. One of the reasons for this is that women are more likely to think that they are imposters - see yesterday's post. Women compete for intimate relationships, but they do not like anyone to lose completely - that is, to be utterly excluded.
My favorite piece of evidence that Susan Pinker cites is a study of what happens when boys and girls run alone, compared to when they run with others. Alone, boys and girls run at the same speed. When boys are running with someone else, especially another boy, they run faster. When girls run with someone else, especially another girl, they will slow down to keep pace with the other.