Across the economy, men make more than women. Part of the push to close the wage gap is to remove barriers to women entering high-paying careers, like those in science, technology, engineering, and mathematics (STEM).

But even at this high level, a new study finds women in STEM careers make 31 percent less then men in their first year after graduation.

Bruce Weinberg, an Ohio State University economist, recently co-authored the study in the American Economic Review: Papers and Proceedings, which draws data from 1,237 new PhDs at four research universities.

The study combined university employee data with census records to provide details about home lives. This (anonymous) data has not been available until now.

Weinberg's study suggests two reasons for the pay gap

It's hard to know whether the pay gap is the result of discrimination, Weinberg says.

What is clear is that 20 percent of that pay gap is due to the fields women and men choose to study. "Women go into biomedical fields, and men go into engineering, math, and computer science," he says. The biomedical fields pay less. Also, women are more likely to take jobs in academia or government than in private industry, which is more lucrative.

"These differences could be due to choice, external forces ... or a combination of both," the study notes.

The researchers account for the rest of the difference by controlling for marital status and children. Married women with children in STEM seem to be paid less.

But "if you look at a man and a woman who are in the same field of study — and neither of them are married and neither of them have children — there would be no difference in earnings," says Weinberg.

This raises some important questions. Are the women being discriminated against for having children (i.e., not being offered opportunities because employers fear the kids will get in the way of productivity)? Or are women choosing less lucrative careers because they make better accommodations for work-life balance?

"We really can't say," Weinberg says of his analysis. But he notes that overall, the results "say something about those jobs."

That something could be that these lucrative jobs are either less accommodating or less welcoming for women with families.

This current analysis just includes PhDs from four research universities (which are kept anonymous), but Weinberg and his colleagues are currently working on a data set with 25 universities. That will make for a more definitive snapshot of what happens to PhDs when they enter the working world. But even the preliminary results from these few schools are worth noting.

Keep in mind: This is data on people just starting out in their careers. Starting at a lower rate makes it harder to move up to a very high income on incremental raises alone.

Careers in science, technology, engineering, and mathematics can take a very, very long time to get off the ground. PhD programs in these fields can last for five years or more, and just as these newly minted doctoral degree holders are entering the working world, they may also be thinking about settling down. These choices seem to only hurt women. In the study, men were the more likely to have children, and they didn't suffer financial consequences.

Women are already underrepresented in the sciences and engineering just in terms of numbers: About 75 percent of the STEM workforce is male. The fact that once women get into this workforce they're still at odds is doubly disheartening.

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