Garett Jones, one of my favourite economists, has a new book out, Hive Mind, about how the IQ of your compatriots is more important in determining your life outcomes than your own IQ. Since the IQs of people in the developing world is recorded as lower than the IQs of people in the developed world, this means that mass immigration could worsen our lives by lowering average IQ, since this feeds through to lower social trust and worse institutions.
This would reduce the value of open borders as a policy regime. But Bryan Caplan, famous libertarian advocate of more liberalised migration laws, disagrees, and they debate this issue here, in what I think is a very high quality and interesting tussle.