An entrance sign written in Spanish is seen along the U.S.-Mexico border fence near Brownsville, Texas on August 4, 2014. (Courtesy Reuters)

A combination of cheap transportation and enormous disparities in income across countries has inspired unprecedented numbers of people to uproot: there are now 230 million people around the world living outside the country of their birth, 46 million of them in the United States. Not surprisingly, immigration tends to flow from poor places to rich ones: in the world’s 18 richest countries, immigrants constitute 16 percent of the population. If one includes those who are descendants of recent immigrants, that percentage is significantly larger and is certain to grow, since immigrants generally have more children than domestic populations. Consider that, in 2010, 13 percent of the U.S. population was born outside the country, yet 24 percent of those younger than 18 had foreign-born parents.

Policymakers in rich countries have tended to treat […]