The summer job is considered a rite of passage for the American Teenager. It is a time when tossing newspaper bundles and bussing restaurant tables acts as a rehearsal for weightier adult responsibilities, like bundling investments and bussing dinner-party plates. But in the last few decades, the summer job has been disappearing. In the summer of 1978, 60 percent of teens were working or looking for work. Last summer, just 35 percent were.

Why did American teens stop trying to get summer jobs? One typical answer is: They’re just kids, and kids are getting lazier.

One can rule out that hypothesis pretty quickly. The number of teens in the workforce has collapsed since 2000, as the graph below shows. But the share of NEETs—young people who are “Neither in Education, Employment, or Training”—has been extraordinarily steady. In fact, it has not budged more than 0.1 percentage point since the late 1990s. Just 7 percent of American teens are NEETs, which is lower than France and about the same as the mean of all advanced economies in the OECD. The supposed laziness of American teenagers is unchanging and, literally, average...