Today, 12 April 2011, is “Equal-Pay Day” in the U.S. If you took the median-salary American man and the median-salary woman, and started paying them both on the first of 2010, today is the day when the woman will have finally earned what the man took in through 31 December, about 14 weeks ago.

Of course, it’s not that simple. You can’t just take any man and any woman and make that comparison. The figure that’s used for this is the median income: take all the men’s annual salaries, list them in order of lowest to highest, then pick the one in the middle. Do the same for women’s salaries. Compare. The median of the women’s salaries is about 78% of the median of the men’s. We could use the average (mean) instead of the median, but for these sorts of economic comparisons it’s typically the median that’s used, because it doesn’t suffer from skewing by the extremes at the edges.

The problem is that the majority of the gap comes from the fact that men and women are not equally represented in all the different jobs... and the jobs that employ primarily men just so happen to pay more than the ones that employ primarily women. I can’t imagine how that happened, but, well, there it is. Nurses earn less than doctors. Beauticians earn less than plumbers. Teachers earn less than corporate executives. And so on.

And it doesn’t stop there: what about college-educated women? What about those with PhDs? Because another fact is that more women than men are finishing college, these days, and more women than men are completing PhD programs. Doesn’t that fix it?

No. For one thing, when we look at the fields that women are getting degrees in, we find the same thing: the fields that attract women more tend to be the less lucrative ones.

But also, when we break it down by field we still find differences. In April of 2007, the American Association of University Women released a study titled “Behind the Pay Gap” (PDF). The study showed that female biological scientists earn 75% of what their male colleagues do. In mathematics, the figure is 76%; in psychology, 86%. Women in engineering are almost there: they earn 95% of what the men do. But less than 20% of the engineering majors are women.

The other argument for why there’s a pay gap is that women and men make different decisions about their lives. Women choose motherhood, a bigger hit against career advancement and salary opportunities than fatherhood. More women work part time. And so on.

The AAUW study looked at that. They controlled for those decisions, and they compared men and women who really could be reasonably compared. They looked at people in the same fields, at the same schools, with the same grades. They considered those of the same race, the same socio-economic status, the same family situations. They didn’t just compare apples to apples; they compared, as economist Heather Boushey puts it, “Granny Smith apples to Granny Smith apples.”

And they found that even in that case, there’s an unexplained pay gap of 5% the year after college, which increases to 12% ten years later. From the study:

The pay gap between female and male college graduates cannot be fully accounted for by factors known to affect wages, such as experience (including work hours), training, education, and personal characteristics. Gender pay discrimination can be overt or it can be subtle. It is difficult to document because someone’s gender is usually easily identified by name, voice, or appearance. The only way to discover discrimination is to eliminate the other possible explanations. In this analysis the portion of the pay gap that remains unexplained after all other factors are taken into account is 5 percent one year after graduation and 12 percent 10 years after graduation. These unexplained gaps are evidence of discrimination, which remains a serious problem for women in the work force.

But the speed of the improvement is not. The disparity of pay between male-dominated fields and female-dominated ones is not. The gap in pay between highly trained men and women in the same field is not. And that unexplained 5-to-12 percent is certainly not.

Let’s keep pushing that date back, and look for the year when equal-pay day is December 31^st.