Facts about occupational licensing
We find that in 2006, 29 percent of the workforce was required to hold
an occupational license from a government agency, which is a higher
percentage than that found in studies that rely on state-level
occupational licensing data.
That is from a new paper by Morris Kleiner and Alan Krueger. I am happy to see such respected economists turning their attention to a neglected issue. Here is a non-gated version of the paper.
There is, by the way, an interesting sentence buried near the end of the paper:
In contrast, union members perceive themselves as less competent than other workers.