The most prominent is a mystery that has proved impervious to easy explanation: why wage increases haven’t been more robust, when the market continues to edge toward full employment.

Friday’s report showed that hourly earnings went up by 2.6 percent over the past year, not much faster than inflation. The subdued wage gains eased the prospect that the Federal Reserve would accelerate its plans to raise interest rates, helping to send stocks higher. But lagging pay also reflects how the economy of 2018 is fundamentally different from earlier eras.

“A 3.9 percent rate today doesn’t suggest as tight a labor market as 3.9 percent in 2000 or 3.9 percent in the late 1960s,” said Ellen Zentner, Morgan Stanley’s chief United States economist.

A lot has changed since the turn of the century. The share of working-age women in the labor force began to fall in 2000, after increasing for decades. Men have been dropping out for much longer. The upshot is that a smaller share of people are participating in the labor market, and it’s easier to get low levels of unemployment when fewer people are vying for jobs.

In fact, a shrinking labor force in April is part of why the unemployment rate fell to 3.9 percent from 4.1 percent even as payrolls grew by a fairly routine 164,000 jobs.

The population is also older than they used to be, on balance. The baby-boom generation has moved steadily toward retirement over the last two decades. And those still working have not helped push wages up. Generally, workers climb the economic ladder fastest when they are young, and so an older work force may weigh on average wages, economists say.

In 2000, wages for rank-and-file workers rose at an annual rate of around 4 percent. Part of the problem now is that some 60 percent of the jobs added since 2010 have been in low-wage, service-sector jobs, according to Morgan Stanley.