This demographic trend has been the subject of much noise and consternation lately. Nicholas Eberstadt, a demographer at the conservative American Enterprise Institute, calls the development a “quiet catastrophe: the collapse, over two generations, of work for American men.”

Eberstadt concedes that he can’t pinpoint the precise causes, but he implies that the problem, at its root, emanates from some kind of moral or societal dysfunction.

“Time-use surveys suggest [these men] are almost entirely idle,” Eberstadt wrote in a Wall Street Journal op-ed a few weeks ago. “Unlike in the past, the U.S. is now evidently rich enough to carry them, after a fashion,” he added.

Princeton professor Alan Krueger, a former chief economist at the Department of Labor and former chairman of Obama's Council of Economic Advisers, has taken a look at the same data — but he came away with a different conclusion.

What stood out to him is that a lot of these men say they are in considerable pain.

In a recently released draft of his paper, which he will present at a Federal Reserve conference in Boston on Friday, Krueger finds that 44 percent of male, prime-age labor force dropouts say they took pain medication the day prior — which is more than twice the rate reported by employed men.

In a follow-up survey focusing on these male labor-force dropouts, Krueger found that these were serious habits. About two-thirds of the people taking pain medication were using prescription drugs, not over-the-counter remedies like Tylenol or aspirin.

Compared to their employed counterparts, these men also reported more emotional pain — more feelings of sadness, tiredness, and stress.

About 20 percent of these men say they have difficulty walking or climbing stairs; about 16 percent say they have memory or concentration problems; over a third say they have a disability of some kind, and nearly 18 percent say they have multiple disabilities.

“This is a group that is particularly unhappy and distressed, that often faces significant health conditions which are a barrier to employment,” Krueger said in an interview.

Declining health is becoming a major reason prime-age men are working less and less. The Bureau of Labor Statistics regularly asks people why they aren’t in the labor force. Of the 11.5 percent of prime-age men who aren’t employed or looking for a job, over half blame illness or disability. The rest are either retired, going to school, or performing housework.

In other words, fully 6 percent of American men between the ages of 25-54 feel that their minds or their bodies are too broken for them to work. This rate has nearly quadrupled since 1968, when only 1.6 percent of men felt the same way.

Women have seen the same increase in pain and disability. About 5.8 percent of prime-age women say they can’t work because of health issues, up from 1.8 percent in 1968. But that trend has been somewhat obscured by the tremendous number of women who have joined the labor force in recent decades.

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Others have also pointed out that America is suffering a pain epidemic. Last year, a widely-cited paper from Princeton economists Anne Case and Angus Deaton found that the death rate has been increasing for middle-aged whites in America. At least part of the problem is that opiate deaths have been increasing — but Case and Deaton also discovered that people are taking painkillers for good reason. The economists found that among whites aged 45-54, over a third report chronic joint pain while a fifth report say they have neck pain — and these rates have been going up in the past decade.

We still don’t know why Americans are in so much pain, but it is clear that sickness and disability have become major obstacles for people who want to work, Krueger says.

If we want to increase labor force participation, we will have to work harder at diagnosing and fixing these health problems. “We should look at interventions to help this group get back on their feet and become healthy enough and maintain their health long enough to start working again,” he says.

The Obama administration has proposed other solutions to coax people back into the labor force — using tax credits or wage subsidies, for instance. These are worthy ideas, but they start from the assumption that these men mainly struggle with finding suitable work. As Krueger's research points out, many of them seem to struggle just to make it through the day.