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An increasing number of student loan borrowers are behind on their payments, creating concern that millions of Americans may never pay off their debts.

About 43% of the 22 million Americans with federal student loans weren't making payments as of Jan. 1, the Wall Street Journal reported. That's actually a slight improvement from the same time last year, when the non-payment rate was 46%, the Department of Education found.

But that drop is largely a result of more borrowers joining a plan for distressed borrowers in order to lower their payments. Enrollment in these programs—which reduce monthly bills by allowing borrowers to pay a smaller percentage of their income—surged 48% since last year.

Additionally, 3.6 million borrowers—about 1 in every 6—were in default on a total $56 billion in student debt, meaning they had gone at least a year without making a payment. Another three million borrowers, owing an approximate total of $66 billion, were behind in payments by at least a month. And yet another three million, owing nearly $110 billion, were in "forbearance" or "deferment," which means they had received permission to temporarily cease payments due to a financial emergency like unemployment.

Advocacy groups and the federal Consumer Financial Protection Bureau blame loan servicers, who are are hired by the government to collect debt, for not doing enough to get in touch with struggling borrowers. However, one company, Navient Corp., says it attempts to reach each borrower between 230 and 300 times—via letters, emails, calls and text messages—in the year leading up to his or her default.

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The numbers, however, may not reflect a completely accurate picture of American student debt: Some defaulted loans may be from previous decades, and the government is limited in its ability to write them off. Still, the statistics have the Obama administration worried, causing it to ramp up efforts to reach borrowers with income-based repayment plans.

The Education Department also has created a "behavioral sciences unit" to study the psychology of why borrowers don't repay their debt. In some cases, the explanation may be very logical; research from Navient shows that borrowers tend to prioritize other bills like car loans over student loans. Indeed, when borrowers fail to pay down auto loans, they run the risk of having their car repossessed; with student loans, there is no such consequence.