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.

There is no contemporary version of the 1870s railroads, the 1920s auto industry or even the 1990s Internet sector. Total economic output over the last decade, as measured by the gross domestic product, has grown more slowly than in any 10-year period during the 1950s, ’60s, ’70s, ’80s or ’90s.

Perhaps the most important reason, beyond the financial crisis, is the overall skill level of the work force. The United States is the only rich country in the world that has not substantially increased the share of young adults with the equivalent of a bachelor’s degree over the past three decades. Some less technical measures of human capital, like the percentage of children living with two parents, have deteriorated. The country has also chosen not to welcome many scientists and entrepreneurs who would like to move here.

The relationship between skills and economic success is not an exact one, yet it is certainly strong enough to notice, and not just in the reams of peer-reviewed studies on the subject. Australia, New Zealand, Canada and much of Northern Europe have made considerable educational progress since the 1980s, for instance. Their unemployment rates, which were once higher than ours, are now lower. Within this country, the 50 most educated metropolitan areas have an average jobless rate of 7.3 percent, according to Moody’s Analytics; in the 50 least educated, the average rate is 11.4 percent.

Despite the media’s focus on those college graduates who are struggling, it’s not much of an exaggeration to say that people with a four-year degree — who have an unemployment rate of just 4.3 percent — are barely experiencing an economic downturn.

Economic downturns do often send people streaming back to school, and this one is no exception. So there is a chance that it will lead to a surge in skill formation. Yet it seems unlikely to do nearly as much on that score as the Great Depression, which helped make high school universal. High school, of course, is free. Today’s educational frontier, college, is not. In fact, it has become more expensive lately, as state cutbacks have led to tuition increases.