Other factors that could be keeping people out of the job market are also being examined. In a widely discussed paper earlier this year, for example, economists at the University of Chicago and other schools argued that some young men are opting out of the labor force to play video games. (Other economists are skeptical.) Some research has pointed a finger at the federal disability system: Nearly two million more Americans are receiving federal disability payments than when the recession began in 2007, an increase that some economists argue is a reflection of the benefits’ use as an alternative to work. Some research, however, has concluded that the increase in disability claimants is due mostly to an aging population and is, in any case, a small piece of the overall decline in employment.

Deeper changes in the structure of the American economy could also be playing a part. A variety of evidence, including declining rates of entrepreneurship and falling job turnover, suggests the nation’s economy has become less dynamic and flexible since 2000, which could have made it harder for workers and companies alike to adapt following the shock of a recession.

And the recession may have accelerated trends that were already underway: Research from Lisa B. Kahn, a Yale economist, and a co-author has found that companies had, in effect, taken advantage of the recession to replace workers with machines. That was particularly damaging for men without a college degree — a group that was already struggling before the recession and that has been especially slow to recover from it.

“We were on a kind of trend where that group of people was going to find it harder and harder to find productive outlets in the labor market, and the recession kind of accelerated that,” Ms. Kahn said. Had the change happened more gradually, Ms. Kahn added, workers might have had a chance to adapt and learn new skills; instead, the recession left millions of them jobless in an environment where there was little demand for their labor.

Work like that of Ms. Kahn and Mr. Krueger suggests that what begins as a cyclical recession-driven problem can harden into a permanent structural issue. That could carry a lesson for policy makers: If the United States no longer recovers as quickly from recessions as it once did, and if those slow recoveries can leave permanent scars on workers, then it is all the more important to kick-start the economy before too much damage is done.

Ben S. Bernanke, then the chairman of the Federal Reserve, warned during the recession and the recovery that workers who stayed unemployed too long might drift too far away from the labor market to return. Mr. Yagan’s research suggests that efforts by the Fed, Congress and the Obama administration to prevent that had not gone far enough.

Jared Bernstein, a former economic adviser to Vice President Joseph R. Biden Jr. who is now with the liberal Center on Budget and Policy Priorities, said Mr. Yagan’s research could be read as an indictment of economic policy after the recession. If Congress and the administration had been willing to act more aggressively, they might have avoided some of the long-lasting damage that has been done.