The humanities labor market is in crisis. Higher education industry trade publications are full of essays by young Ph.D.s who despair of ever finding a steady job. Phrases like “unfolding catastrophe” and “extinction event” are common. The number of new jobs for English professors has fallen every year since 2012, by a total of 33 percent.

In response to these trends and a longer-term decline in academic job security, the Democratic presidential candidate Bernie Sanders has made a proposal. In exchange for federal funding to reduce public college and university tuition to zero, he said, at least 75 percent of college courses would have to be taught by tenured or tenure-track professors. Currently, that proportion is less than 40 percent and dropping.

How this happened is a story of a rupture in the way the academy produces and consumes people with scholarly credentials.

In 1995, roughly 940,000 people were employed teaching college. Of those, about 400,000 had tenure or were on track to get it. They enjoyed professional status, strong job security, relatively good pay (on average), and the freedom to speak their minds.

The rest were so-called contingent or adjunct faculty: some employed full time, others filling in a course or two per semester. They had lower pay, less status and tenuous job security, particularly if they spoke their minds. There were also thousands of graduate students, not counted in the numbers above, teaching as part of their training. (The University of California, Santa Cruz, which is known to be progressive even by the standards of academia, recently fired 54 graduate assistants who were striking for higher pay.)

The percentage of professors on the tenure track had been slowly declining since the 1970s. In the late 1990s came a demographic event that would ultimately throw the university labor market into a tailspin: the first college years of the so-called millennials, those born from the early 1980s to the mid-1990s.