Collier is best known for his writings on “the bottom billion,” the world’s poorest people. In this book, he explores the economics and politics of global migration and offers a surprising and controversial case for restricting it. Owing to the global growth of economic inequality, huge numbers of people in poor countries are eager to leave their difficult conditions in search of better lives elsewhere. Although migrants often manage to improve their own lots, Collier argues that the countries they leave behind and the countries that receive them might actually become worse off. He acknowledges the economic benefits receiving countries enjoy from immigration but believes that after a certain point, these benefits are outweighed by the social costs generated by a decline in “mutual regard”: that is, the trust and social cohesion necessary for a well-functioning society. For the countries the migrants leave behind, Collier also sees losses, associated with the brain drain and weakened national identity. Collier wisely limits himself to cautious and hedged arguments, because, as he himself warns, debates about restrictions on immigration can quickly get tangled up with the politics of racism.