[W]hen we compare and contrast the economic performance in these 22 [right-to-work] states against the others, we find interesting things. For example, from 1999 to 2009 (the last such year for which data are available), the aggregate real all-industry GDP of the 22 right-to-work states grew by 24.2 percent, nearly 40 percent more than the gain registered by the other 28 states as a group.
Even more dramatic is the contrast if we look at personal income growth. From 2000 to 2010, real personal incomes grew by an average of 24.3 percent in the 22 right-to-work states, more than double the rate for the other 28 as a group. But the strongest indicator is the migration of young adults. In 2009, there were 20 percent more 25- to 34-year-olds in right-to-work states than in 1999. In the compulsory union states, the increase was only 3.3 percent—barely one-sixth as much.
-From "The Right to Work: A Fundamental Freedom" by Mark Mix, President, National Right to Work Legal Defense Foundation in Imprimis: a Publication of Hillsdale College.
(Right-to-Work states protect a worker's right not to join a union, except for employees who work in the railway or airline industries and who work in certain federal related situations.)
A map identifying the right-to-work states and the forced-unionism states is here.