AMERICANS are famously footloose. Migration rates across state lines are more than twice those between Canadian provinces. Mobility is an economic asset: it leads to better matches between workers and jobs and helps America absorb economic shocks as people shift in search of employment. Since the early 1990s, however, when roughly 3% of Americans moved from one state to another each year, the rate of gross interstate migration has fallen by about half. That makes economists fret: a nation of stick-in-the-muds could face disappointing productivity growth and more stubborn unemployment than it is used to.