From a paper by Peterson, Pandya, & Leblang (March 2013):
We show that over the period 1973-2010 states with greater physician control over licensure requirements imposed more stringent requirements for migrant physician licensure and, as a consequence, received fewer new migrant physicians. By our estimates half of all US states could resolve their physician shortages within five year just by equalizing migrant and native licensure requirements. This article advances research on the political economy of immigration and highlights an overlooked dimension of international economic integration: regulatory rent-seeking as a barrier to the cross-national mobility of human capital, and the public policy implications of such barriers.
The title of the paper is "Doctors with Borders: Occupational Licensing as an Implicit Barrier to High-Skill Migration."