The unexpectedly large number of American workers who piled into the Social Security Administration's disability program during the recession and its aftermath threatens to cost the economy tens of billions a year in lost wages and diminished tax revenues.

Signs of the problem surfaced Friday, in a dismal jobs report that showed U.S. labor force participation rates falling last month to the lowest levels since 1979, the wrong direction for an economy that instead needs new legions of working men and women to drive growth...