Immigration is an issue that elicits heated views from all sides of the political and economic spectrum. In the 21st century, how might we expect our lives and societies to be affected by changes in immigration? In Part One of a two-part series, economist Lant Pritchett argues that there are five irresistible forces setting the world up for a new wave of mass migration. Topping the list are the huge cross-national inequalities in wages and standards of living. Differences in labour demand across countries comprise another pressure promoting migration.