Exodus, by Paul Collier

Exodus: Immigration and Multiculturalism in the 21st Century, by Paul Collier, (Penguin, RRP£20/RRP$27.95)

This book opens with a surprise. Paul Collier, professor of economics and public policy at Oxford, describes a framed picture on his desk of Karl Hellenschmidt, a penniless German immigrant to Britain in the early 20th century.

But when the first world war began, Hellenschmidt was interned as an enemy alien, his wife fell into “terminal depression” and their son Karl Jr was “pulled out of school” to run the family’s shop. He would go on to change his name to Charles Collier – and a family was absorbed into mainstream society. The surprise is not that Prof Collier descends from immigrant stock, but that he opens this academic study with a personal anecdote. His history mirrors the under­lying narrative of this book.

For years, Prof Collier writes, rational debate on immigration has been impossible because “policy has been fought over using competing values rather than competing evidence”. The debate was split between the extremes of those who viewed migration as a threat and those who believed that to question it was tantamount to racism.

The debate has shifted, led by left-leaning thinkers such as Robert Putnam, the Harvard political scientist, and David Goodhart, author of The British Dream , who have studied the impact of immigration on host societies, and whether diversity improves or threatens cohesiveness. Further, what impact should this have on policy towards migration?

Prof Collier, who has spent his career studying poor countries, has decided to step into this delicate territory. Exodus, he writes, is not an effort to tell us what to think about immigration but an attempt to create a new framework for how we think about it. For too long policy makers on both sides of the political divide have been asking the wrong question. Rather than arguing about whether immigration is good or bad, they should focus on the optimum degree of diversity. To understand this requires studying the rate at which migrants are absorbed into mainstream society.

He has created a model he believes is a more reliable way of achieving this goal. It relies on three factors: the size of the diaspora; the idea that migration increases the diaspora’s size whereas “absorption into mainstream society reduces it”; and the idea that “the rate of absorption depends on the size of the diaspora”.

Why does this matter? First, the bigger the diaspora, the more slowly it is absorbed into mainstream society. “The diaspora is the accumulated stock of unabsorbed migrants, so it is the diaspora that measures the impact of migration on diversity,” he writes. “That in turns puts strains on relations between the indigenous population and newcomers.” Second, understanding the rate of absorption provides policy makers with a more reliable tool for planning for future migration so that they can establish limits when necessary.

Every state, he contends, must aim to achieve equilibrium. For recipient nations, this would enable governments to manage the indigenous population’s concerns over competition for jobs and resources (although it is not clear when, or even whether, “absorbed” migrants become “indigenous”). Should the economy suffer a setback, there would be enough slack in the system to prevent immigrants becoming a target of resentment. In this context, Prof Collier’s research finds migration economically positive.

For states of origin, achieving the right balance is about maximising the degree to which they can benefit from their best and brightest going abroad to study, and the impact remittances can have in supporting the economy. Get this wrong, and students who leave do not come back and the value of remittances relative to the loss of talent will diminish.

While Prof Collier poses interesting questions, his findings do not seem surprising, and many sentences include tentative phrases such as “might” and “could be”, which undermine the strength of his evidence-based approach.

Curiously, Prof Collier also seems to misread signs that on-the-ground writers and journalists have documented well. For example, he writes of the French as being for over a century “proselytisers for their national culture, and that substantial migration has been compatible with a continuing sense of pride”. This of a nation gripped by riots in the immigrant-dominated banlieues in 2005, and where the extreme right, anti-foreigner National Front is shaping the debate over immigration and achieving electoral success.

Prof Collier’s is a voice to which it is worth paying attention. His book could be better written but this grandson of an immigrant is asking important questions about one of the world’s most pressing issues.

The writer is deputy editor of FT Weekend Magazine