Among the lowlights of the jobs report for April was the news that the share of adults who are either working or looking for work fell. For men, this measure — called the labor force participation rate — was at its lowest level since 1948, when the government first began keeping track.

I’ve received a number of e-mails from readers asking whether this decline can be attributed to the wave of baby-boomer retirements.

Yes, as America ages, its overall labor force participation rate will fall because older people are less likely to work. But even excluding older Americans, labor force participation rates have still fallen sharply over the last few decades, and especially in the last five years.

This chart shows the share of Americans 25 to 54 years old who are involved in the labor force, either by working or actively looking for work:

As you can see, this age group is also dropping out of the labor force. The participation rate fell after the 2001 recession, never recovered, and then started another slide that began in the financial crisis. This trend among prime-working-age Americans cannot be explained by baby boomers’ retiring.

You may notice that the labor force participation rate had been climbing from the 1940s through about 1990. That rise reflects the fact that more women entered the labor force as gender roles evolved. Women’s labor force participation rate continued rising through the late 1990s, dropped a couple of percentage points, and then more or less flat-lined.

The main reason the labor force has been declining in the last couple of decades, then, is that men have been dropping out in droves. Here is a chart of labor force participation for workers 25 to 54, but showing men only:

Source: Bureau of Labor Statistics

Where are all these men going, and how are they supporting themselves, if they’re not in the work force? My colleagues David Leonhardt and Louis Uchitelle looked into this question a few years ago.