What are the benefits and costs of a diverse population? How should a liberal democracy define the limits of multiculturalism? In answering these questions, the high-income countries will also define what kind of society they wish to be in the 21st century.

As the World Bank’s latest Economic Prospects report makes clear, the pressure for migration from poor to rich countries is a permanent feature of our integrating world.* The share of migrants in the populations of the high-income countries rose from 4.4 per cent in 1960 to 11.4 per cent in 2005. It seems certain to rise still further, given both demographic trends and the persistent gaps in incomes and wages between developing and high-income countries.