If these indirect benefits do matter, it may explain why Mr Prasad and company failed three years ago to prove a link between the migration of capital and growth.

The International Organisation for Migration says it's planning an operation to evacuate thousands of foreigners from the Libyan capital Tripoli as fighting gets closer to the city.

Migration narrows the gap in human capital among the cities, especially the gap between Shanghai and the other 15 cities of the Yangtze River Delta.