Missing: nearly 3 million American workers.

This isn't the latest dystopian science fiction story or the pitch for a Hollywood thriller. It is the economic trend that has alarmed experts and policymakers alike.

A growing number of people — many in their prime working years — have simply given up on landing a job. On Friday, the Labor Department reported that the overall unemployment rate dropped slightly to 6.1 percent in August. But that improvement was mostly a result of Americans dropping out of the labor force, not of their finding work.

The situation helps explain why so many ordinary Americans remain doubtful about the recovery, now in its fifth year.

Adding to the discontent, the pace of job creation in August also slowed. Employers added 142,000 workers to their payrolls, the smallest number this year; it was the first time since January that monthly job creation fell below the 200,000 level.

It is tempting to discount weaker hiring in any single month, and August tends to be especially unpredictable. As millions of people take their summer breaks, a smaller proportion of Americans respond to government data collectors during the month, said Tara M. Sinclair, an economist at Indeed.com, one of the nation's largest sites for job postings.

Economists at Bank of America Merrill Lynch noted that August payroll growth had been revised upward in 12 of the last 15 years by an average of 31,000. "The report was clearly disappointing and contrasts with the otherwise strong economic data we have seen recently," the bank said in a note to clients. "We advise not overreacting given the volatility of nonfarm payrolls and possibility of an upward revision."

But the fall in the labor participation rate cannot be dismissed as an aberration. The rate dropped to 62.8 percent last month, the lowest level since the late 1970s.

Although the shrinking proportion of Americans in the workforce is often attributed to the retirement of baby boomers, 25- to 54-year-old workers are quitting as well. Just over 81 percent of this group was in the workforce last month, compared with 83.4 percent in early 2007, equivalent to the disappearance of nearly three million workers.

"The structure of the U.S. economy has shifted," said Dean Maki, chief U.S. economist at Barclays. "Some of these people dropped out when the labor market was bad, and it's not clear how many will come back."