One influential study finds that when a U.S. state hits economic difficulties, workers quickly begin leaving for other states; within six years the unemployment rate falls back to the nation average. This compares with a lifetime of 15 or 20 years for a typical specialized machine, and perhaps 50 years a shopping mall or office building. So labor in certainly a less specific factor than most kinds of capital. On the other hand, highly trained worker are pretty stuck with their craft: A brain surgeon might have made a pretty good violinist, but she cannot switch careers in mid-lift.