As a rule, out-of-work men are less educated than the population as a whole. Their numbers have grown sharply among black men and men who live in hard-hit industrial areas like Michigan, West Virginia and upstate New York, as well as those who live in rural states like Mississippi and Oklahoma.

The missing men are also more likely to live alone. Nearly 60 percent are divorced, separated, widowed or never married, up from 50 percent a decade earlier, the Census Bureau reports. Sometimes women who are working throw out men who are not, says Kathryn Edin, a sociologist at the University of Pennsylvania. In any case, without a household to support, there is less pressure to work, and for men who fall behind on support payments, an incentive exists to work off the books — hiding employment — so that wages cannot be garnisheed.

“What happens to a lot of guys who become unmoored from family life, they become unmoored from everything,” Ms. Edin said. “They are just living without attachments and by the time they are 40 or 50 years old, the things that kept these men from falling away — family and community life — are gone.”

Even as more men are dropping out of the work force, more women are entering it. This change has occurred partly because employment has shrunk in industries where men predominated, like manufacturing, while fields where women are far more common, like teaching, health care and retailing, have grown. Today, about 73 percent of women between 30 and 54 have a job, compared with 45 percent in the mid-1960’s, according to an analysis of Census data by researchers at Queens College. Many women without jobs are raising children at home, while men who are out of a job tend to be doing neither family work nor paid work.

Women are also making inroads in fields where they were once excluded — as lawyers and doctors, for example, and on Wall Street. Men still make significantly more money than women, but as women become more educated than men, even more men may end up out of the work force.

At the low end of the spectrum, men emerging from prison with felony records are not easily absorbed into steady employment. Hundreds of thousands of young men were jailed in the 1980’s and 1990’s, in a surge of convictions for drug-related crimes. As prisoners, they were not counted in the employment data; as ex-prisoners they are. They are now being freed in their 30’s and 40’s and are struggling to be hired. Roughly two million men in this group have prison records, according to a calculation by Richard Freeman and Harry J. Holzer, labor economists at Harvard and the Urban Institute, respectively.Many of these men do not find work because of their records.

Despite their great numbers, many of the men not working are missing from the nation’s best-known statistic on unemployment. The jobless rate is now a low 4.6 percent, yet that number excludes most of the missing men, because they have stopped looking for work and are therefore not considered officially unemployed. That makes the unemployment rate a far less useful measure of the country’s well-being than it once was.