It's hard to overstate how far women have come in the last century. They are now almost as active in the labor market as men, and equally or even better educated. They account for about half of all law and medical school enrollments, and lead men in fields such as biological sciences, pharmacy and optometry.

Still, women have yet to reach the same level of pay. As of 2010, the annual earnings of the median full-time, full-year female worker stood at 77 percent of the median male's — up from 56 percent in 1980 but still far from parity. For college graduates, the number was an even lower 72 percent.

Why the persistent difference? U.S. data provide two clues. First, the gap increases with age: Women start their careers close to earnings parity with men, then fall behind over the next several decades. Second, wage differences are concentrated within occupations, meaning that women earn less not because they choose lower-paid professions.