WASHINGTON - When the Labor Department announces May employment figures today, the jobless rate will likely be about 5 percent, half the level reached in the 1981-82 recession.

Given that home builders aren't hiring, auto companies are laying off legions and Americans lack confidence in the economy, why isn't unemployment higher?

Economists cite several factors but some think one often gets overlooked: Prisons are keeping so many people out of the labor market that the true jobless rate is partly masked.

"Incarceration rates have gone up enormously in the last 20 years," said Rebecca Blank, an economist and senior fellow at the Brookings Institution, a centrist research group. "We're jailing a lot more people now," which keeps them from competing for the low-skilled manufacturing jobs that have grown scarce, she said.

From 1980 to 2006, the number of prisoners in this country jumped from 420,000 to more than 2 million, Blank said. A recent study by the Pew Center on the States found that one in every 100 adult Americans is now incarcerated.

In 2006, the most recent year Blank's research covers, the 4.6 percent U.S. unemployment rate would have been 5 percent if people behind bars and in the armed forces had been seeking private-sector jobs and therefore counted in the unemployment statistics, she said. Among black men, the rate would have pushed up from 9.6 percent to 10.6 percent.

"We have disproportionately taken young men of color out of the work force" with much stiffer penalties for drug law violations, Blank said.

A 2005 study in the American Journal of Sociology said that by 1999, more than 40 percent of young black male high school dropouts were in prison or jail, compared with 10.3 percent of young white male dropouts.

But not everyone agrees that the nation's high incarceration rate has an impact on employment. James Sherk, a fellow at the Heritage Foundation, a conservative research group, says the unemployment rate is low simply because the labor market remains tight, with many employers seeking even low-skilled workers.

If people now incarcerated under tough drug laws had never been put into prison, the great majority of them could have found jobs if they had wanted them, he said. Attributing low unemployment to high incarceration is "grasping at straws," he said.

Peter Morici, a University of Maryland economics professor who describes himself as "a pretty conservative guy," agrees with Blank that tougher sentencing policies help mask a weak job market, especially for people lacking higher educations.

Sentencing drug users to prison "does matter," Morici said. "If these people were in the general population, they would contribute to a higher rate of unemployment."