The Wall Street Journal had an article yesterday about the emptying out of the middle of the US. Controlling for ethnicity, the picture below shows in the darkest shades of red the greatest declines in the white population from 2000 to 2010:
What if we had a law that everybody had to stay in their home state? What if North Dakotans had to stay in North Dakota despite the collapsing economy there? Then wages would collapse and we would have very poor North Dakotans. Happily no one would dream of such a stupid law. Instead we have middle class North Dakotans moving to other places voluntarily, where employers want to hire them voluntarily. And so (former) North Dakotans stay middle class.
For states...but not for countries. We treat migration usually as a non-option if Zambia has an economic decline, so Zambians stay there and get even poorer as the economy declines.
This is the great point made by Lant Pritchett in a classic article and in a CGD book. Why can't we start treating Zambians like North Dakotans? If their home economy is declining, let them move to other places voluntarily, where employers want to hire them voluntarily. Why do we recognize the right to live wherever you want for North Dakotans and not for Zambians?
Response to David in comment below: yes things could be good for the North Dakotans left behind BECAUSE OF all the North Dakotans that have left. Just think of supply and demand for labor -- if the spur to migration was a downward shift in demand for labor, then having a lot leave shifts back the supply of labor and wages can remain the same. WITHOUT migration, North Dakota wages would have collapsed, which is what actually does happen in the migration-not-allowed Zambias of the world.