But seemingly small missteps can send a borrower spiraling into default. The loan companies have misdirected payments, lost paperwork and charged the wrong interest rate, the Consumer Financial Protection Bureau and Government Accountability Office have shown.

Say a borrower has fallen behind on her monthly payments, and her loan company calls to check in. The rules are complicated, and the responses of an individual call agent are critical.

One agent might guide her into enrolling in an income-based repayment plan that resets her payment to zero, with the balance eventually forgiven. Even if she qualifies for the income-based plan, a different agent might put her onto a less desirable path: what is known as forbearance, with her payments suspended, interest mounting and no progress made toward ending her obligation.

In a carefully designed study published in January, Daniel Herbst, a doctoral student in economics at Princeton University, examined thousands of records at a large student-loan servicer. He found that the arbitrary process of assigning a call to one operator instead of another has real effects on a borrower’s loan status. Simply being placed in an income-based repayment program slashed loan delinquency by 21 percentage points to essentially zero. Those enrolled in the income-based program were 2 percentage points more likely to hold a mortgage (a substantial increase given the baseline of 20 percent), likely reflecting an increase in homeownership. And they were paying down their student loans faster than those who were enrolled in standard plans, in which payments are fixed for 10 years and do not vary with income.

All this points to lower default rates for people on income-based plans. In such plans, payments are zero for the very lowest earners and a percentage of income for those with higher pay. Reducing default matters, because it can have devastating personal consequences, including damaged credit ratings and reduced job and housing prospects because employers and landlords check credit reports.