1) Are they in school?

This would be one of the most benign—or even hopeful—reasons for the drop in male participation. Alas, it doesn’t seem to explain much.

Since 1990, the number of men over the age of 25 enrolled in a post-secondary institution has increased slightly, from 2.5 million to about 3 million, most of which was an increase among people enrolled part-time. Overall, a jump of 500,000 accounts for just a fraction of the growth of non-working men. (If the male participation rate hadn’t changed since 1990, there would be about 3 million more men in the labor force.)

The men most likely to drop out of the labor force today are those who never started college. This is a remarkable shift. Fifty years ago, college graduates and high-school dropouts were similarly likely to work, as one can see in the graph below. Today a high-school graduate who has never gone to college is four times more likely to drop out of the labor force than he was in 1964.

Men between 25 and 54 are much better educated than they were in 1964. That fact alone should have predicted a rising participation rate, since college graduates are more likely to work. Instead, the least educated men are abandoning the work force more than ever. That is the real mystery.

2) Are they on disability?

This is another common explanation for the drop in male participation. But again it doesn’t explain more than a fraction of the phenomenon.

There’s not much doubt that Social Security Disability Insurance takes people out of the workforce, often by inelegant design. In order to qualify for disability payments, people typically have to prove that they cannot work full-time. SSDI critics say this policy sidelines many people who might otherwise be able to contribute to the economy.

But how many people does SSDI really remove? From 1967 to 2014, the share of prime-age men getting disability insurance rose from 1 percent to 3 percent. There is little chance that this increase is entirely the result of several million fraudulent attempts to get money without working. But even if it were, SSDI would still only explain about one-quarter of the decline in the male participation rate over that time. There are many good reasons to reform disability insurance. But it’s not the singular driving force behind the decline of working men.

3) Do stay-at-home dads account for the change?

This is an easy one: no.

First, married men are more likely to work than non-married men. Second, fathers have stayed in the workforce more than non-fathers. Third, more than 75 percent of prime-age men not in the workforce do not have a working spouse. Fourth, time-use surveys of non-working men have found that they are less likely to be caring for household members than working men. They do, however, watch more than twice as much television.