Here's one reason why women earn less than men - they do more housework. Mark Bryan estimates that, for married women, each hour of housework per week reduces wages by 0.16%.
As the average married woman does 12 and a half hours of housework (five more than single women), this implies a wage reduction of 2%.
This is controlling for the possibility that women with high earnings capacity tend to be more career-oriented and less inclined to do housework than other women.
The effect seems to exist independently of whether women have children or not, so it operates on top of the "child penalty (pdf)" in women's earnings.
Dr Bryan says:
Tiring activities like housework reduce the amount of effort available for market work, resulting in lower prductivity and wages.