According to Warren Farrell in Exploiting the Gender Gap – New York Times women make less than men by choice. According to Farrell, women choose to work fewer hours, travel less and occupy safer occupations. Single, childless women make 177% of their childless male counterparts and "people who work 44 hours a week make, on average, more than twice the pay of someone working 34 hours a week."
More food for thought.
P.S. Take all his statistics with a grain of salt. I don’t think he’s always comparing apples to apples.