Similarly, in 1970, only about 2 percent of firefighters had a college degree, compared with more than 15 percent now, Vedder and his colleagues found. And, according to research by economists Paul Harrington and Andrew Sum of Northeastern University, about 1 in 4 bartenders has some sort of degree.

Beaudry and his colleagues say that such change has been driven by a decline in the demand for highly skilled work — the opposite of the conventional wisdom about such demand. The employment rate in “cognitive” occupations — managerial, professional and technical jobs — increased markedly from 1980 to 2000, their research found, but it has since stagnated, even as the supply of skilled workers has continued to grow.

What has changed? One possibility, as I’ve previously written, is that the effects of a globalizing workforce are creeping up the income scale. Many jobs that once required cognitive skill can be automated. Anything that can be digitized can be done either by computer or by workers abroad. While the “winner take all” phenomenon may still mean extremely high returns for workers at the very top, that may be relevant for a shrinking share of college graduates.

Whatever the explanation, the Beaudry team argues that an excess of skilled workers has led them into the “routine” job market — such as sales and clerical jobs — reducing wages there and pushing less skilled workers into “manual” jobs in construction, farming and so on.