Economists have identified a variety of reasons for sluggish Japanese income gains -- and some of those have faint echoes in the U.S.

The number of non-regular Japanese workers -- those who fill part-time, temporary and contract jobs -- has grown about 40 percent since 2002. They earn less than permanent, full-time employees and have less bargaining power to win pay increases.

In the U.S., close to 25 percent more people were working part-time last year than in 2002, even though they preferred to have a full-time job. And just like in Japan, they found it difficult to boost their earnings.

About seven in 10 U.S. part-time workers didn’t get a pay increase in the past year, based on the Fed’s 2017 Survey of Household Economics and Decisionmaking. That compares to just 40 percent of full-time workers -- a divide that is similar to other data, according to Atlanta Fed senior policy adviser John Robertson.

Limited labor mobility is another cause of Japan’s muted income gains, according to economists at the International Monetary Fund, who tied that to the country’s system of lifetime employment. Workers who jump to new positions usually earn more.

In the U.S., the share of Americans moving is the lowest on record, according to the White House’s Council of Economic Advisers.

“The geographic immobility of workers is a conundrum with no easy solution,” it said in its annual economic report released on Feb. 21, adding that may be causing some Americans to stop looking for a job.

Another big shift in labor market dynamics common to both countries stems from their aging populations, though that’s happening much more quickly in Japan than in the U.S. Older workers often earn more. So when they retire, that depresses the average salary.

“The so-called Silver Tsunami will continue to be a drag on aggregate wage growth for some time,” San Francisco Fed economist Mary Daly and her colleagues wrote in a blog post last year, referring to the continued “large-scale exit of higher-paid baby boomers from the labor force.”

U.S. companies also are increasingly rewarding their workers with bonuses rather than via increases in base salaries, although they still lag far behind their Japanese counterparts in following that practice.