Morris M. Kleiner writes,

Even in the 1950s, licensing covered less than 5% of the American workforce. Now more than 20% of the U.S. workforce is covered by state licensing laws…prices in regulated occupations have increased more–and the earnings of practitioners have become higher–than in comparable occupations with similar levels of human capital and experience.

This appeared in the op-ed section of the weekend Wall Street Journal, but I’ll be darned if I can find it on line anywhere.

UPDATE. Spencer, in a comment, gave this link to the article. Thanks.
Here is an article by Kleiner that is available on his web site.