We recently came across a study showing that lesbians -- at least in the UK -- earned consistently more than their hetero counterparts. You can fill in your own explanation. It turns out there's been a lot of research lately into this question, much of it undermining the argument that plain vanilla bigotry accounts for the male/female disparity (although we haven't heard that argument since we attended a friend's graduation from Mt. Holyoke). Here's the latest we've come across:
In most Western countries illness-related absenteeism is higher among female workers than among male workers. Using the personnel dataset of a large Italian bank, we show that the probability of an absence due to illness increases for females, relative to males, approximately 28 days after a previous illness. This difference disappears for workers age 45 or older. We interpret this as evidence that the menstrual cycle raises female absenteeism. Absences with a 28-day cycle explain a significant fraction of the male-female absenteeism gap.
The study goes onto suggest this difference accounts for a 12% income gap.
Hmm, perhaps this whole freakonomics thing thing has run its course. Sorta makes you pine for the days when economists did stuff like forecasting the GDP, or advise presidents about price controls.
More about menstruation and the gender earnings gap [New Economist]