The one apparent bright spot in Friday’s report — a sharp drop in the unemployment rate to 6.7 percent from 7 percent — was tarnished because it largely resulted from people exiting the work force rather than because they landed jobs. The work force shrank by 347,000 in December, reversing a big gain from November, and returning the proportion of Americans in the labor force to its October level of 62.8 percent, the lowest in 35 years.

While some of that decline is because of demographic factors like an aging population and rising retirements, Ms. Coronado said she was particularly troubled by how many prime-age workers were dropping out.

Among workers aged 45 to 54, the participation rate dropped 0.4 percentage point to 79.2 percent, the lowest since 1988. For workers 55 and older, the participation rate edged down only 0.1 percentage point. “It just keeps dropping and dropping,” she said. “It’s depressing, as it’s not just older workers retiring.”

After initially dropping in the wake of the Labor Department report Friday morning, stocks recovered later in the day as investors shifted their focus away from the labor market to what they hope will be more buoyant results as companies report fourth-quarter earnings in the next few weeks.

Some economists, impressed by other recent data showing steadily rising economic output, private surveys showing healthier payroll gains, a growing manufacturing sector, and increased exports, suggested that December’s figures represented a statistical fluke rather than another of the so-called swoons that have been a recurring feature of the fitful recovery that has followed the Great Recession.

“My advice is to ignore this number,” said Nariman Behravesh, chief economist at IHS. “A lot of other indicators are showing strength. It was largely noise last month, and the Fed will see it the same way, unless there is other evidence that gives them pause.”

Most experts say the Federal Reserve will stick with its plan to gradually taper its stimulus program when policy-makers meet later this month, but future reductions could be delayed if payroll gains remain weak in February and March.