There’s a debate among economists about why a college degree is worth so much. That the credential is valuable is not in doubt. According to the Pew Research Center, college graduates earn about $17,500 more annually than high school grads. Why?

The “human capital” school believes that students learn valuable skills in college that employers are willing to pay for. The “signaling” school doubts that the content of a college education is really that marketable. They argue that employers are interested in the traits — diligence, intelligence, self-control — that a degree reflects.