At $1.3 trillion, America’s pile of student loan debt is near-incomprehensible in size.

“I’m not smart enough to say what’s going to happen with the crisis, but there’s a day of reckoning coming,” bestselling author John Grisham, who is turning the crisis into the plot of a a thriller novel, commented late last year. Those who might be considered ”smart enough”—the economists, analysts, and researchers who’ve studied the issue intensely in recent years—have been keeping a wary eye on the swelling figures, and have come to a bleak conclusion. A new report from Brookings, a think tank, suggests that the default rate on student loans could rise a lot higher than previously expected.

Most research on student debt examines default rates three to five years from when repayment begins, arriving at an average default rate of 11.5%. Brookings used a new dataset to assess default rates over a longer time horizon. Applying trends from older cohorts to newer ones, the study found that default rates could rise as high as 40% by 2023.

The study looked at the default rate for all first-time college students who started university in 1995 and 2003, following them from the moment they matriculated through to 2015; the analysis revealed that default rates barely taper off even a decade after starting school. What’s more, 10 years after taking out a loan, around 14% of all 1995 entrants had defaulted—after 20 years, that figure rose to 25%.

Things are even worse for the 2003 cohort, with over 25% defaulting by the 11-year mark. Judith Scott-Clayton, a Columbia University economist who authored the report, concludes that if the 2003 cohort follows previous patterns, they’ll reach a 40% default rate at the 20-year mark. (And that applies to larger debts—while 52% of the 1995 group took out federal loans, 60% of the group of 2003 students did.)

What’s going on? The rise of for-profit colleges, many of which have been publicly accused of deceiving students into debt, is one leading cause. More than twice as many of the students who started in 2003 attended for-profit schools, versus their counterparts who enrolled in 1995.

Another cause is simply record-high tuition figures that colleges across the country—for-profit, public, and private—charge for an education these days. It is now common for a bachelor’s degree to cost upwards of $200,000. With a college degree a near necessity for getting a foothold in the workforce, Americans have little choice but grin and bear it, and many are falling into debt as a result.

For those deciding whether to take on student loans, there are ways to minimize the debt. Choosing one’s university carefully helps. Scott-Clayton also notes that borrowing a larger amount of money is an indicator that a person is less likely to default, not more: Those from the 2003 cohort who borrowed less than $6,125 proved more likely to default than those borrowing more than $24,000. This is likely because borrowers who took out larger federal loans did so for more expensive educational institutions, which tend to offer more value, in quality and employment prospects, for the price.