In this week's Economist there's a review of a book called Economic Facts and Fallacies by Thomas Sowell. I quote:
"Today, never-married, childless, university-educated American women of between 40 and 64 earn $7,000 more than similar men. No explanation for this positive gap is forthcoming. But at the very least, it suggests that, where women do earn less, it is due more to rational factors (suche as their unwillingness to work long hours) than to sexism."