It’s International Women’s Day, which makes today a good day to examine why America just can’t seem to pay women as much as men.

To be sure, the gender pay gap has been  shrinking for decades. But, by one measure, it got worse last year, the  Institute for Women’s Policy Research reports. In aggregate, women working full-time earned 80.9% of what men earned in 2012; in 2011, that ratio stood at 82.2%.

There are some disagreements about these figures. The White House has said that women earn 77 cents for every dollar men earn; on the other hand, writes  Suzy Khimm , if you totally disregard life choices, like having kids, that number hits 91 cents.

But the plain truth is that women often get paid less for doing the exact same jobs as men. Planet Money recently compiled a list of the jobs with the  biggest gender pay gaps . They found that female insurance salespeople, financial advisors, physicians and surgeons get paid roughly 60-70% of what their male counterparts do. On the other end of the spectrum, female counselors are paid 2.6% more than men.

Though the gender pay gap tends to narrow when you compare similar jobs, as the  St Louis Fed notes, there’s a still a depressingly 1950s breakdown of what professions men and women enter:

Men are more likely to be lawyers, doctors and business executives, while women are more likely to be teachers, nurses and office clerks. This gender occupational segregation might be a primary factor behind the wage gap.

Lisa Pollack has a nice two-part guide to some of the other thorny studies on gender and pay. She highlights one study which finds that women get paid less because they don’t ask for raises, though the study looked only at administrative assistant jobs. Another study found that women get more raises than men, but it only examined small companies.

One thing we do know is that gender inequality in pay is pervasive. It cuts across race, and it applies to both hourly and full-time workers,  Dylan Matthews notes. It also starts as soon as women  graduate from college. Since 1979, the BLS says, women have gone from earning roughly 60% of what men earn, to somewhere near 80%. Still, “men’s economic privilege has been dented rather than eroded,” Mathews writes.