In the four years since the Great Recession officially ended, the productivity of American workers — those lucky enough to have jobs — has risen smartly. But the United States still has two million fewer jobs than before the downturn, the unemployment rate is stuck at levels not seen since the early 1990s and the proportion of adults who are working is four percentage points off its peak in 2000. This job drought has spurred pundits to wonder whether a profound employment sickness has overtaken us. And from there, it’s only a short leap to ask whether that illness isn’t...