With unemployment at 3.8 percent, its lowest level in many years, the labor market seems healthy.

But that number hides a perplexing anomaly: The percentage of men who are neither working nor looking for work has risen substantially over the past several decades.

The issue, in economist’s jargon, is labor force participation. When the Bureau of Labor Statistics surveys households, every adult is put into one of three categories. Those who have a job are employed. Those who are not working but are searching for a job are unemployed. Those who are neither working nor looking for work are counted as out of the labor force.

This last group is ignored when calculating the unemployment rate. The presumption is that if a person without a job isn’t looking for one, then he or she doesn’t want one, and the joblessness is not a problem. But is that really accurate?

The data show some striking changes over time. Among women, the share out of the labor force has fallen from 66 percent in 1950 to 43 percent today. That is not surprising in light of changing social norms and the greater career opportunities now open to women.