Still, the reasons for concern today are serious. Even before the financial crisis began, the American economy was not healthy. Job growth was so weak during the economic expansion from 2001 to 2007 that employment failed to keep pace with the growing population, and the share of working adults declined. For the average person with a job, income growth barely exceeded inflation.

The closest thing to a unified explanation for these problems is a mirror image of what made the 1930s so important. Then, the United States was vastly increasing its productive capacity, as Mr. Field argued in his recent book, "A Great Leap Forward." Partly because the Depression was eliminating inefficiencies but mostly because of the emergence of new technologies, the economy was adding muscle and shedding fat. Those changes, combined with the vast industrialization for World War II, made possible the postwar boom.

In recent years, on the other hand, the economy has not done an especially good job of building its productive capacity. Yes, innovations like the iPad and Twitter have altered daily life. And, yes, companies have figured out how to produce just as many goods and services with fewer workers. But the country has not developed any major new industries that employ large and growing numbers of workers.