A big puzzle looms over the U.S. economy: Friday's jobs report tells us that the unemployment rate has fallen to 6.7% from a peak of 10% at the height of the Great Recession. But at the same time, only 63.2% of Americans 16 or older are participating in the labor force, which, while up a bit in March, is down substantially since 2000. As recently as the late 1990s, the U.S. was a nation in which employment, job creation and labor force participation went hand in hand. That is no longer the case.

What's going on? Think of the labor market as a spring bash you've been throwing with great success for many years. You've sent out the invitations again, but this time the response is much less enthusiastic than at the same point in previous years.

One possibility is that you just need to beat the bushes more, using reminders of past fun as "stimulus" to get people's attention. Another possibility is that interest has shifted away from your big party to other activities.

Economists are sorting out which of these scenarios best explains the slack numbers on labor-force participation—and offers the best hope of reversing them. Is the problem cyclical, so that, if we push for faster growth, workers will come back, as they have in the past with upturns in the business cycle? Or do deeper structural problems in the economy have to be fixed before we can expect any real progress? To the extent that problems are related to retirement or work disincentives that are either hard to change or created by policy, familiar monetary or fiscal policies may have little effect—a point getting too little attention in Washington.