The US _has_ been mostly self-sorting for decades now. It's one of the reasons politics is so polarized right now and Congress is incapable of governing without a looming emergency.
It's never as easy as just looking at the outcomes. As with any type of sociology, it's not possible to force large sets of people to do identical things for valid control+experimental groups.
Methods of taxation, tax rates, size of state government, expectations of government services, natural resources, macroeconomic conditions, talent of the population, and natural economic benefits all distinguish different states. There's really no effective way to control for the large variety of conditions across many states.
It's never as easy as just looking at the outcomes. As with any type of sociology, it's not possible to force large sets of people to do identical things for valid control+experimental groups.
Methods of taxation, tax rates, size of state government, expectations of government services, natural resources, macroeconomic conditions, talent of the population, and natural economic benefits all distinguish different states. There's really no effective way to control for the large variety of conditions across many states.