One measure that accounts for both the unemployed and nonparticipants is the employment-to-population ratio. Unlike the unemployment rate, it has not returned to its 2001 level. But when adjusted to account for demographic changes, it shows a tight relationship with wage growth. This is what we might expect to see if slack not captured by the unemployment rate were still a drag on earnings.

Another idea is that weaker wage growth primarily reflects a slowdown in productivity gains. Janet Yellen, the former Fed chair, espoused this view in a speech last year. If the value of what workers produce is growing more slowly than in the past, we may expect this to be reflected in smaller raises. And while productivity data is noisy, analysis based on the same approach Ms. Yellen used shows that the trend in productivity growth is down more than a percentage point from its 2001 pace.

This slowdown is its own economic mystery, particularly since research shows that the phenomenon is present globally across advanced economies.

The Current Population Survey does not track worker productivity, but it does show that occupations of all skill requirements — even high-skilled ones — are seeing wage growth down from 2001.

In fact, when we rank different jobs by their skill level using data from the economists David Autor and David Dorn, we find that the highest-skilled occupations — which we would expect to be more productive — typically saw the highest wage growth in 2001. But in 2018, that is no longer the case.

This suggests that if the productivity slowdown is a factor in slow wage growth, it’s a pervasive one across occupations.