Perhaps not surprisingly, grad students tend to take on more debt when going into fields where the pay is higher.

Students studying medicine and law typically borrow more than $100,000 to get through school, and many go on to high-paying careers.

At the other end of the spectrum, many Ph.D. students wind up in academia. Most get grants and subsidies — and the majority don't have to borrow any money at all to get through grad school.

One striking case: MBAs. People who go to business school take on significantly less debt than people at other professional schools. Most MBA programs are two years long — shorter than law school (three years) or med school (four).

But that's not nearly enough to explain the difference.

I asked Steve Tadelis, a professor of economics at UC Berkeley's Haas School of Business, about this. He pointed out that a lot of students who wind up going to business school work in high paying fields like consulting or engineering right out of college.

That makes it easier for them to save up for grad school. So they pay more of the tuition up front, and don't borrow as much.

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Darian Woods is currently pursuing his MPP at the Goldman School, and interned at NPR's Planet Money during the summer of 2015.

Find the original NPR Planet Money article here.