Exodus: How Migration is Changing Our World. By Paul Collier. Oxford University Press USA; 309 pages; $27.95. Allen Lane; £20. Buy from Amazon.com, Amazon.co.uk

PAUL COLLIER is one of the world’s most thoughtful economists. His books consistently illuminate and provoke. “Exodus” is no exception. Most polemics about migration argue either that it is good or bad. They address the wrong question, says Mr Collier. The right one is: how much more migration would be beneficial, and to whom?

He examines this question from three perspectives: the migrants themselves, the countries they leave and the countries to which they move.

Migration makes migrants better off. If it did not, they would go home. Those who move from poor countries to rich ones quickly start earning rich-country wages, which may be ten times more than they could have earned back home. “Their productivity rockets upwards,” says Mr Collier, because they are “escaping from countries with dysfunctional social models”.

This is a crucial insight. Bar a few oil sheikhdoms, rich countries are rich because they are well organised, and poor countries are poor because they are not. A factory worker in Nigeria produces less than he would in New Zealand because the society around him is dysfunctional: the power keeps failing, spare parts do not arrive on time and managers are busy battling bribe-hungry bureaucrats. When a rich country lets in immigrants, it is extending to them the benefits of good governance and the rule of law.

What of the countries that receive immigrants? Mr Collier argues that they have benefited from past immigration, but will probably suffer if it continues unchecked.

So far, immigrants have typically filled niches in the labour market that complement rather than displace the native-born. For most citizens of rich countries, immigration has meant slightly higher wages, as fresh brains with new ideas make local firms more productive. It may have dragged down wages for the least-skilled, but only by a tiny amount.

However, says Mr Collier, continued mass immigration threatens the cultural cohesion of rich countries. Some diversity adds spice: think of Thai restaurants or Congolese music. But a large unabsorbed diaspora may cling to the cultural norms that made its country of origin dysfunctional, and spread them to the host country. Furthermore, when a society becomes too heterogeneous, its people may be unwilling to pay for a generous welfare state, he says. Support for redistribution dwindles if taxpayers think the beneficiaries will be people unlike themselves.

Finally, Mr Collier looks at the effect of emigration on poor countries. Up to a point, it makes them better off. Emigrants send good ideas and hard currency home. The prospect of emigration prompts locals to study hard and learn useful skills; many then stay behind and enrich the domestic talent pool instead. But if too many educated people leave, poor countries are worse off. Big emerging markets such as China, India and Brazil benefit from emigration, but the smallest and poorest nations do not: Haiti, for example, has lost 85% of its educated people.

Mr Collier’s most arresting argument is that past waves of migration have created the conditions under which migration will henceforth accelerate. Emigration is less daunting if you can move to a neighbourhood where lots of your compatriots have already settled. There, you can speak your native language, eat familiar food and ask your cousins to help you find a job. Because many Western countries allow recent immigrants to sponsor visas for their relatives, Mr Collier frets that large, unassimilated diasporas will keep growing. And as they grow, they will become harder to assimilate.

Mr Collier is plainly not a bigot and his arguments should be taken seriously. Nonetheless, he is far too gloomy. He lives in Britain, which is nearly 90% white and has seen substantial immigration only relatively recently. His worries are mostly about the harm that immigration might do, rather than any it has already done. Indeed, the evidence he marshals suggests that so far it has been hugely beneficial.

It is possible that Britain will prove unable to cope with greater diversity in the future, but one cannot help noticing that the most diverse part of the country—London, which is less than 50% white British—is also by far the richest. It is also rather livelier than the lily-white counties that surround it.

America’s population consists almost entirely of immigrants and their descendants, yet it is rich, dynamic, peaceful and united by abundant national pride. Every past wave of newcomers has assimilated; why should the next one be different? The recent history of Canada, Australia and New Zealand also suggests that large-scale immigration is compatible with prosperity and social cohesion.

Mr Collier is right that there is a tension between mass immigration and the welfare state. A rich country that invited all and sundry to live off the dole would not stay rich for long. Immigrants assimilate better in America than in most European countries in part because welfare is less generous there. In parts of Europe it is possible for able-bodied newcomers to subsist on handouts, which infuriates the native-born. In America, by and large, immigrants have to work, so they do. Through work, they swiftly integrate into society.

Mr Collier approves of the European-style welfare state, so his policy prescriptions are aimed largely at preventing immigration from undermining it. He would peg the number of immigrants to how well previous arrivals have integrated. He would welcome quite a lot of skilled migrants and students (a good idea) but curb family reunions (which sounds harsh). He would allow in asylum-seekers from war zones but send them back when peace returns to their homelands. (This, he explains, would help their homelands rebuild themselves.) As for illegal immigrants, he would offer them the chance to register as guest workers who pay taxes but receive no social benefits.

Insisting that immigrants work is sound policy, but the tone of “Exodus” is problematic. Mr Collier finds endless objections to a policy—more or less unlimited immigration—that no country has adopted. In the process, he exaggerates the possible risks of mobility and underplays its proven benefits.