Another factor in the declining participation is that the oldest baby boomers have reached retirement age.
But Craig Alexander, chief economist at TD Bank Group, says "demographics cannot explain the amount of decline" in labor force participation.
Many Americans without jobs remain so discouraged that they've given up on the job market. Others have retired early. Younger ones have enrolled in school.
Some Americans have suspended their job hunt until the employment landscape brightens. A rising number are collecting disability checks.
"It's not necessarily people retiring," Bovino says. "It's young people going back to school" rather than taking their chances on a weak job market.
Labor force participation for Americans ages 16 to 19 was just 34 percent last month. That's near their record low of 33.5 percent set last year.
It isn't supposed to be this way. After a recession, a brightening economy is supposed to draw people back into the job market. But it hasn't happened. Labor force participation "certainly shouldn't be at current levels," Alexander says.
There aren't enough jobs being filled. Employers are hiring about 4.3 million people a month -- before layoffs, dismissals and resignations. In 2007, before the Great Recession, they were hiring 5.2 million a month.
There are three unemployed people, on average, competing for each job opening, compared with 1.8 when the recession officially began in December 2007.