Collier drew attention a few years back with The Bottom Billion, which called for rescuing the global poor by unconventional means (including military intervention) that he cast as evidence of his tough-mindedness. In Exodus: How Migration Is Changing Our World, Collier is even more eager to present himself as an enemy of orthodoxy. Like most economists, he thinks the benefits of migration have generally exceeded the costs; unlike many, he sees cause for worry should migration significantly rise. He’s arguing against colleagues who say that letting more people from poor countries work in rich ones is the single best way to reduce global poverty. (If you want to help someone from Mali, they urge, let him mow your lawn.) Dismissing their approach as ethically “glib,” Collier warns that rapid ethnic change can threaten fragile social bonds and weaken support for the welfare state—imperiling the “fruits of successful nationhood” that migrants seek.

It’s the rare economist who calls an economic perspective a “woefully inadequate” guide to action and then switches the subject to culture. In part, Collier’s approach reflects the wariness that even many left-leaning Europeans feel after the past decade, which included the Amsterdam murder of Theo van Gogh, the London bus and subway bombings, and the Paris riots. But immigration alarms Americans, too (and always has). From progressives anxious about the marginalized poor to conservatives worried about Hispanic conquest and Sharia law, pessimistic voices can be heard across the political spectrum. Yet the more Collier dwells on what could go wrong, the more I found myself appreciating how much in the U.S. has gone right over the past four decades. If “two cheers for us” seems Pollyannaish for a precarious work still in progress, how about a cheer and a half?

The global story, in Collier’s telling, begins after World War II, when three “golden” decades of rapid growth brought fabulous wealth to rich countries and left other countries further behind. Wider wage gaps gave the poor more reason to move, and cheaper travel and communications sped the way. The big winners have been the migrants themselves, many of whom have been able to multiply their earnings five or 10 times. Collier sees a mixed impact on the countries they have left. Remittances have brought in billions of dollars, but in the smallest and poorest countries, he thinks the benefits have been outweighed by brain drain.

Countries that take in migrants have received modest economic rewards overall, he says, along with the joys of diversity—fresh thinking, chicken tikka masala. But Collier cautions that diversity can also have “corrosive effects” on trust, cooperation, and the willingness to redistribute income. He highlights research by Harvard’s Robert Putnam, of Bowling Alone fame, who has found that diversity reduces trust not only between ethnic groups but also within them. “Inhabitants of diverse communities tend to withdraw from collective life, to distrust their neighbors, regardless of the color of their skin, to withdraw even from close friends … and to huddle unhappily in front of the television,” Putnam has written. Arguing that migration left unchecked will continue to accelerate, Collier insists that rich countries must set limits or put their “critical achievements … at risk.”