Women across professions from financial specialists to podiatrists to physicians and surgeons to aircraft pilots are earning less than three-quarters of their male counterparts’ salaries. There’s no question workplace equality has made great strides over the years, still there’s something holding men and women back from earning equal pay. What could it be?

There are plenty of speculations for why the disparity persists: Women don’t ask for more money. Others believe discrimination is the cause. Still others, that employers have different promotion standards for men and women. There is also the undeniable effects that off-ramping to have kids has on women’s careers.

Having children is indeed a contributing factor to how much income men and women earn. A study of men and women in New York City published this May found that men with children had significantly higher incomes than women with children and men and women without children. Researchers from City University of New York where the study was done call this the “Mommy Tax” and the “Daddy Bonus.”

But it’s not a Great Depression era bonus-for-the-family-man mentality that leads to this discrepancy. “Its not that daddies are getting paid more,” says Harvard economist Claudia Goldin. “It’s that the same exact men receive more because they just get their act together. They say, ‘Hey, I’m a father; I have to do something about it.'”

One area in which men might not be stepping up their game quite as much is in adapting well to changes in the job market, specifically in middle-skill jobs, according to research by David Autor out of the MIT Department of Economics. When the number of men and women in middle-skill jobs dropped from 1979 to 2007, females moved upward into higher-level jobs at a rate much greater than their male counterparts.

While the percentage of women making up traditional female occupations like teaching and nursing has gone down over the years, women have also increasingly taken more steps to get the education and training they need in fields like law and medicine in order to move up in the ranks. “Looking forward, it is clear that females will be the more educated sex for many years to come,” writes Autor.