Critics have long criticized for-profit schools for recruiting unqualified students who go on to have higher loan default rates than their peers. In their defense, for-profit schools point out that they tend to attract more economically disadvantaged students from less educated families.

college graduation (AP/Stephan Savoia)

A new study from two Boston University economists has controlled for these socio-economic differences and finds that students at for-profit schools fail to receive any wage boost from obtaining a certificate or associate's degree. "There is little evidence of a return to any certificate or degree from a for-profit," the researchers write in a new paper for the National Bureau of Economic Research

By contrast, students who receive degrees "from a public or not-for-profit institution receive a large wage premium," they explain, boosting their earnings by as much as 15 percent. The same doesn't hold true for non-degree certificate programs, with one exception: certificate programs in health fields boost wages, but only if they are from non-profit or public universities, the paper concludes.

(The Washington Post owns Kaplan, which runs a for-profit college service.)