A reader counters the others:
Of course women with PhDs are likely to earn less than men with PhDs. Women are overwhelmingly earning PhDs in liberal arts, while men earn more PhDs in hard science and business. Hard sciences and business PhDs pay a lot more, for two simple reasons (and neither of them gender-related):
(1) If you earn a PhD in one of those fields, you can be employed by something other than a university (like a Fortune 100 company, a tech start-up, or the research lab at a major defense contractor), and (2) supply and demand: there are far fewer people earning science/business PhDs, so if a university is hiring, they have to offer a competitive salary (as opposed to an English department which can probably choose among hundreds of PhDs for a single position).
Dr. Warren Farrell wrote a masterful book, Why Men Earn More, which explains the perceived "wage gap"- 98% of the wage gap can be explained by choices people make in college and in careers. For example, men are far more likely to: work in dangerous professions, work outdoors, commute long hours, work 50+ hours a week, travel at work’s behest, and enter fields that are less personally satisfying (e.g., accountant vs. kindergarten teacher). All of these choices increase the salary men earn over women for jobs requiring the same amount of education.
Further, many young women will choose a career path in college (when they are 19) in anticipation of balancing work and family demands that may not arise for a decade or more. For example, they may choose a safer, less risky career track like Human Resources rather than Finance, when they would be better served aggressively climbing the corporate ladder until and if they are actually faced with such a choice between work and family demands (see this excellent TED talk by the COO of Facebook, Sheryl Sandberg).
The real culprit seems to be that few US businesses offer true family-friendliness; it is NOT that businesses are sexist. After all, it would be really inefficient to discriminate against half the labor pool, as companies that did not discriminate would gain an immediate upper-hand in the search for talent.
The Dish 