New research challenges the conventional wisdom that the unemployment rate is falling because workers have given up looking for a job and have exited the labor force, and that the rate likely will climb again once these discouraged Americans renew their search for a job.

In a March 1 report titled “Dispelling an Urban Legend,” economist Dean Maki at Barclays Capital, part of the British financial giant Barclays, argued that the size of the U.S. work force is shrinking as aging baby boomers hit retirement age amid a sluggish economy. This — not the so-called missing workers from the labor force — may be knocking down the jobless rate faster than expected.

“Based on our reading of the evidence, the conventional view that in recoveries the unemployment rate will stop falling and even start to rise because of surging labor force participation rates amounts to something of an urban legend,” Maki and his colleagues concluded. “Such an event has not happened in the past and we do not believe it will this time either.”

The jobless rate has fallen for five consecutive months and now stands at 8.3 percent ahead of Friday’s release of February jobs numbers by the Labor Department. A key private-sector gauge of employment, the ADP National Employment Report, was released Wednesday and showed a better-than-expected 216,000 private-sector jobs added last month.

If the research from Barclays Capital is right, President Obama’s re-election chances could get a big boost. Already, the falling unemployment rate has helped bring up his approval numbers in opinion polls, and gauges of consumer confidence are turning more positive.

Maki’s findings are not necessarily a great development, however, because implicit in a shrinking labor force participation rate is slower economic growth. A smaller labor force means a smaller economy, and Barclays Capital expects sluggish growth in the range of 2.5 percent this year and next.

In an interview, Maki said he expects the unemployment rate to keep falling this year, to end the year at about 7.8 percent. Next year the rate would fall to about 7 percent, according to his projections. That’s considerably better than the Federal Reserve Board’s January central tendency forecast, which put the unemployment rate at the end of this year between 8.2 percent and 8.5 percent, and next year between 7.4 percent and 8.1 percent.

“Really what this (study) is saying is, it is a lot easier to push the unemployment rate down than it used to be,” said Maki.

During the economic expansion from 2003 to late 2007, the labor force participation rate was about 66 percent. It fell amid the recession and has been trending around 64 percent. That has led to the view that the dynamics will change when workers who exited the labor market feel confident enough to start looking for work again.

But Maki thinks the onslaught of retiring boomers — Americans born between 1946 and 1964 — will continue to pull down the labor force participation rate even as discouraged workers resume looking for jobs. The crux of his argument is that the labor force is growing more slowly, and that by itself is holding the jobless rate down.

Barclays’ research found, in fact, that unemployed re-entrants to the work force as a share of the labor force stood at 2.2 percent in January 2012, compared with 1.3 percent in August 2007, months before the December start of the Great Recession. That means re-entrants to the work force are already twice that of pre-recession levels, and yet the unemployment rate has fallen from a high of 10 percent in October 2009 to 8.3 percent in January 2012.