A recent paper from economists Charles Baum and Christopher Ruhm found that for a cohort of kids who had jobs in the late 1970s, working for 20 hours a week in the senior year of high school yielded an 8.3 percent wage boost over their non-working high school buddies. For those who had jobs two decades later, in the late ’90s, the boost was only 4.4 percent. This was true even when the researchers controlled for family background characteristics and student ability.

The most painful part: “Senior year employment was predicted to decrease the probability of subsequently working in the relatively low-paid service sector for the 1979 cohort but to increase it for the 1997 cohort,” they write. They also found that the wage boost in the earlier cohort was largely limited to women, for reasons they could not determine.

According to the U.S. Census, 3.2 million high school students work some kind of job—whether it’s summer, part-time, or full-time. This is not the majority: 71 percent of high school students do not work.