So says Alfred Tella, here:
Fully two-thirds of the 0.9 percentage point drop in the unemployment rate [from November 2010 through February 2011] was due to the decline in labor force participation. If the participation rate had behaved normally, the unemployment rate would have declined by only 0.3 point between November and February, to 9.5 percent last month [instead of 8.9].
He shows this has happened before, too, in 1950, 1958 and 1983.
