Mommy Penalties, Daddy Bonuses

Neil H. Buchanan has just concluded some research on the ways parenthood effects male and female lawyers' salaries, and he has posted a synopsis of his findings over at Feminist Law Professors. Basically, it seems as though men who are fathers earn more than their colleagues while women who are mothers earn less:

My tentative results confirm the “daddy bonus” that others’ have found in other studies, with the range of estimates suggesting a 15-20% salary advantage for fathers. Unlike previous studies, however, I also find a strong suggestion that women with children endure a “mommy penalty,” earning perhaps 10-15% less than the childless (and thus 25-35% less than fathers). I also find some weaker statistical support for the hypothesis that childless women earn less than childless men, with my estimates suggesting an 8-9% difference disfavoring women.

Buchanan goes on to speculate a bit about what causes fathers to benefit from a 'daddy bonus' while mothers suffer from a 'mommy penalty.' Do fathers tend to shirk parenting responsibilities by putting in longer hours while mothers are expected to pick up the parenting slack? Do men simply wait a bit longer to have children and therefore have a salary that matches with their greater experience during the fatherhood years, giving them a pay advantage over young men (and -inexplicably- all women)? Are lawyer fathers more likely to have stay-at-home or part-time spouses that take care of all domestic concerns?

He has already accounted for "differences such as part-time status, the ages of children, and whether the children are living with the lawyer-parent," but Buchanan intends to continue his research and is asking for reader input. If you have ideas of other evidence he should consider, visit him here.