As a general rule, American men earn more than American women. This holds true across industries, across education levels, and across states. But, there's at least one big exception: part-time employees.

In 2012, women who worked less than 40 hours a week generally out-earned men who spent the same amount at their jobs. If they worked 35 hours a week, they earned about 11 percent more. If they worked 25 hours a week, they earned about 4 percent more.

But once females hit that magic 40 hour threshold, their relative pay plummeted, as shown on this graph adapted from a new report by the Bureau of Labor Statistics.

So why do women beat out men while working part-time and lose out while working full-time? My guess is that much (though certainly not all) of the explanation boils down to two reasons: marriage and children. Women, including highly educated professionals, tend to cut their hours once they have families, especially if their husband has a higher salary. Men, meanwhile, are more likely to keep working a full week. And so part-time women, as a group, are somewhat more likely to have gone to college, and far less likely to have dropped out of high school, than part-time men, who may well be working shorter shifts for lack of better options.