​Where have all the working men gone?

A curious thing happened on the way to the 2010s.

Since the late 1940s, American men have been leaving the workforce, disappearing in a steady decline that picked up speed during the Great Recession. After World War II, about 87 percent of adult men held jobs or were looking for work. As of July, that had slipped to 69.3 percent.

Even as the economy recovers and unemployment eases, men still aren't returning to the workforce. The male labor force participation rate is almost three percentage points below where it was in 2009, according to the Federal Reserve Bank of St. Louis.

This puzzling disappearance raises the question of why men are leaving the workforce and not coming back: Is it due to a declining quality of jobs, less educational attainment among men or a result of more gender parity, with men opting out of the workforce because their spouses are earning money?

"The big decline in men's labor force participation since 1993 has been among men under age 55," Heidi Shierholz, an economist at the left-leaning Economic Policy Institute, said. "One thing I often think is driving the labor market is job quality. Job opportunities, wages and how much you get out of working will have a big impact on whether you are in the workforce."

The American labor market has been changing drastically for decades. Manufacturing's importance to the U.S. economy peaked in 1953, when factories contributed more than one-quarter of the country's GDP, according to the National Journal. By 2009, that had shrunk to 11 percent. At the same time, industries that tend to employ more women than men, such as education and social services, have become a more significant part of the nation's economy.

During the same period, women have entered the work force in droves, driving up the female labor force participation rate from just 32 percent in 1948 to about 57 percent currently. To be sure, the recession hit every age group and both genders, with women's labor force participation also slipping since economy's nosedive.

But when it comes to "missing workers," or potential workers who are neither working nor actively seeking work because of weak job opportunities, men between 25 to 54 years old are the biggest slice of the pie, according to an EPI study. That group has 1.85 million "missing" workers, compared with 1.38 million women in the same age range.

Declining job quality doesn't provide an entirely satisfactory answer, Shierholz noted. During the 1960s and 1970s, when job quality was improving, men's participation in the labor market still continued to dip, she said.

In the post-recession era, new jobs are being created, but they're mainly in low-wage industries such as fast-food restaurants and retail chains, according to the National Employment Law Project. While men certainly work in those industries, women tend to dominate some low-wage jobs such as waiting tables. In 46 states, women represent more than half of all minimum-wage workers, according to the National Women's Law Center.

Some men may find it less attractive or harder to find work in an environment that has shifted from manufacturing to the jobs that are proliferating in the post-recession environment, such as stocking shelves or waiting tables.

There's also the growing education gap, with women receiving the lion's share of bachelor's degrees during the past two decades. That has allowed women to fit into a labor market with more rigorous demands, Massachusetts Institute of Technology economist David Autor told NPR.

"You might say, 'Why haven't men responded more effectively, why haven't they educated better, more, why haven't they moved into higher-wage occupations?' " Autor told the site. "And that is less clear."

Then there's the question of whether men are opting out of the workforce as a lifestyle choice.

While still a small number, the U.S. now has more stay-at-home dads than ever, with 2 million opting not to work outside the home in 2012, according to Pew Research. That's almost double the 1.1 million fathers who stayed at home in 1989.

Why fathers say they're giving up work outside the home demonstrates that some might not be choosing the role: Almost one-quarter said they are unable to find work, compared with only 15 percent of stay-at-home mothers.

"If it's people's choices, that's fine," Shierholz noted. "If we see a dropping labor force in part due to weak job opportunities, that's bad overall for the individual and for the macro economy."