Lotsof people have student loans: more than 45 million people. They collectively owe about $1.6 trillion.

That is, of course, a lot of debt — but amid all the national debate right now about what to do about it, it's important to remember that not all debt is created equal, and some borrowers are struggling more than others.

"The large debts we hear about are often taken out by graduate students — people who get an MBA or who get an M.D. or get a law degree or get a master's," says Susan Dynarski, an economist at the University of Michigan.

Those aren't the folks to worry about, Dynarski says. Neither are borrowers who got their bachelor's degree — who on average have about $30,000 in loans after graduation. For many of those borrowers, the loans did their job: They allowed students to go to college, get their degrees, land a better job and, ultimately, pay back those loans.

The people who are really struggling, experts say, are the roughly 1 million borrowers who default on their student loans each year — about 7 million borrowers in total at the end of 2018, according to the latest numbers from the U.S. Education Department.

Defaulting "is not the only sign of struggle, but it's the worst sign of struggle," says Ben Miller, vice president for postsecondary education at the left-leaning Center for American Progress.

When you're in default, the government can take your tax refund or part of your paycheck. When you get older, you can even lose part of your Social Security.

Theseare the people, experts say, that give us a clear idea of who is struggling the most with student debt. And the size of those loans is smaller than you might think: "The typical defaulter has under $10,000 in debt," Miller says.

Borrowers with debt and no degree

"The people having problems with their debts are those who dropped out of school after just a few courses or a year," Dynarski says.

The default rate among borrowers who didn't complete their degree is three times as high as the rate for borrowers who did complete. When these students stop taking classes, they don't get the wage bump that graduates get that would otherwise help them pay back their loans.

"Getting a degree really does make a difference," says Tiffany Jones, the director of higher education policy at the Education Trust.

There are other inequities in the distribution of loans and defaults, too.

Half of African American borrowers who took out loans for the 2003-2004 school year had defaulted after 12 years, according to federal data. Because black students have less generational wealth on average, experts say, they're more likely to borrow in the first place. They're also more likely to attend for-profit schools, and they often earn less money after college.

Even African American borrowers who graduate with a bachelor's degree still default about four times more often than their white counterparts.

"In other words, the bachelor's degree can't completely wipe away issues related to race," Miller says.

Low-income students

Students who receive a Pell Grant — that's the program that provides free money for low-income students — are also more likely to default.

Dynarski explains it this way: If you are a low-income student and you take out loans in addition to Pell Grants, but then drop out and don't earn a degree, then you probably aren't getting a wage bump to help you pay back those loans.

"If you look at the likelihood that someone is going to default, it actually drops as debt goes up," Dynarski says. "That sounds completely counterintuitive, but that's because the missing piece here is earnings. You can't pay off a debt if you don't have any money."

People who went to for-profit colleges

Though for-profit institutions only serve about 10% of students, these students are more likely to default.

When the government looked at the default rates for student borrowers, they found it was nearly double at for-profits what it was at community colleges: of defaulters, just 26% started at community college, while 52% attended a for-profit institution.

For-profit schools are more expensive than community colleges, so students who attend them are more likely to borrow. For-profits also have low graduation rates, so lots of students who start there do not finish, and of those who do, the credentials are less valuable. With that in mind, when you look at all students — not just borrowers — who attend a for-profit college or university, they are four times more likely to default on their loans than community college students.

As Dynarski puts it, often students who attended for-profit colleges have "little education, lots of debt." That's because "the for-profits have very high default and very high dropout rates."

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