http://www.scribd.com/doc/295155616/What-Do-Unions-Do-for-the-Middle-Class
"Several studies show that the decline of unions has been a leading contributor to rising wage inequality, which is closely linked to the hollowing out of the middle class described above. Bruce Western of Harvard University and Jake Rosenfeld of the University of Washington have recently shown that up to one-third of the rise in male wage inequality between 1979 and 2010 is explained by the decline of union membership. Studies by David Card of the University of California, Berkeley, and Thomas Lemieux and W. Craig Riddell of the Vancouver School of Economics at the University of British Columbia; John DiNardo of the University of Michigan, Nicole Fortin of the Vancouver School of Economics, and Lemieux; and Richard Freeman of Harvard University have reached similar conclusions."
