From "The Metrics of the Physician Brain Drain", New England Journal of Medicine, Volume 353:1810-1818, Number 17, October 27, 2005. (The PDF version is here.)
The article does not state any conclusions about the factors that give rise to this result. Of course, my own guess is that the medicine is relatively more free (i.e., less socialized) in the US than in those other three countries, thus making it a more desirable place for doctors to practice and live.
