Among economists, there is near-universal acceptance that immigration generates huge benefits. Inconveniently, from a rhetorical perspective, most go to the migrants themselves. Workers who migrate from poor countries to rich ones typically earn vastly more than they could have in their country of origin. In a paper published in 2009, economists estimated the "place premium" a foreign worker could earn in America relative to the income of an identical worker in his native country. The figures are eye-popping. A Mexican worker can expect to earn more than 2.5 times her Mexican wage, in PPP-adjusted dollars, in America. The multiple for Haitian workers is over 10; for Yemenis it is 15.

No matter how hard a Haitian worker labours, he cannot create around him the institutions, infrastructure and skilled population within which American workers do their jobs. By moving, he gains access to all that at a stroke, which massively boosts the value of his work, whether he is a software engineer or a plumber. Defenders of open borders reckon that restrictions on migration represent a "trillion dollar bills left on the pavement": a missed opportunity to raise the output of hundreds of millions of people, and, in so doing, to boost their quality of life.