A smaller share of both part- and full-time college students are working than 10 and even 20 years ago, according to recently released data from the National Center for Education Statistics.

Given the rising cost of college and skyrocketing student debt, you might think that more students would be working as a way to offset those financial burdens. So, what’s going on?

For answers, Fortune turned to Nicole Smith, chief economist at the Georgetown University Center on Education and the Workforce.

But first, a caveat: The NCES data reflects only a snapshot of the college student community—individuals aged 16 to 24. The CEW’s own study on working student earners points out that one-third of all people enrolled in college with jobs are over 30 years old.

With that distinction in mind, Smith says that college student employment was hit hard during the recession, as evidenced by the dip in the share of college students with jobs starting in 2008. During the recession, “all employment levels froze,” she says, but the “last hired, first fired” approach that many employers took took a major toll on college students. “A student might be working as additional help; as an intern,” Smith says. “When faced with the hard questions, employers get rid of temporary, part-time workers.” That trend popped up in the CEW’s data on student employment.

Even five years later, college student workers are still recovering from the effects of the recession, partly because older Americans in desperate need of employment have taken the jobs students might typically fill—those in food service or retail, Smith says. As evidence of that point, Smith pointed to the so-called underemployment rate among all workers, which counts unemployed Americans, workers who want a job but have stopped looking, and those who have a job but would like to work more hours. Despite the shrinking national unemployment rate, the underemployment figure is still relatively high. In October, it dipped below 10% for the first time since the recession.

Despite the ballooning expense of attending college, we shouldn’t necessarily encourage students to work more as a way to combat higher costs since, at a certain point, jobs can begin to interfere with academic performance. A 2007 study found that there’s an “optimal work-college balance.” Students who worked between 10 and 19 hours per week outperformed their nonworking and working peers in terms of hours studied and grades earned. Logging more hours than that—on top of taking classes and any family responsibilities—is too much of a strain on student workers, Smith says, and such students could end up not completing college on time.