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...