Immigration: people move freely, but who pays? The reason for acceleration is straightforward. The act of migration is costly: the migrant is making an investment. But as diasporas build up, they act as a welcoming committee for further migrants, lowering the costs of migrating. David Cameron's decision to deny Romanian and Bulgarian immigrants welfare benefits for their first three months should help by making immigration a little more costly. Acting pre-emptively, before diasporas have accumulated, makes the change as effective as possible. But in the longer term the right to migrate within Europe for work or benefits should be modified. The free movement of peoples within Europe is a great achievement, but as with the free movement of goods it is rooted in the ethical principle of reciprocity. Germany exports to France without trade barriers; France exports to Germany without trade barriers, and that is mutually beneficial. Several hundred thousand French citizens live in Britain; several hundred thousand Britons live in France: both societies gain from this enhancement of freedom. By elevating free movement of goods and people into principles, we have avoided the petty bureaucracy that would be required to make reciprocity a mathematical requirement - think what "only allow in as much as they allow in" would imply. The accession of south-eastern European countries to the European Union is not just an expanded opportunity for mutual benefit; it is an important contribution to stabilising these societies and enabling them to catch up. But an exodus of their workforce to Britain is a long way from practical reciprocity. It is an inadvertent consequence of a different objective - stabilising Western Europe's neighbourhood. Nor, indeed, is an exodus of the brightest and most energetic young workers from Romania and Bulgaria conducive to their convergence. It is therefore in the interest of Europe as a whole that the rules of movement be modified to recognise the distinctive issues posed by the accession of countries that are much poorer. Mr Cameron's suggestion that rules might be linked to successful income convergence is a doubly good idea. Not only does it accord with the fundamental economic driver of migration, it provides a clear objective by which the governments of the accession countries can be judged by their own citizens. Last week, the European Bank for Reconstruction and Development, whose core mission is to assist the accession countries to converge, reported that economic policy reform had stalled. Its chief economist, Erik Berglöf, issued a plea for a renewed reform effort that would re-start convergence. Those European governments that preside over economies which do not offer credible hope of convergence are failing their people, and other European countries should not be required to provide an alternative home. Britain will not be alone in seeking revision of the rules. All the northern European countries are concerned: for example, earlier this month I was invited to the Dutch Ministry of Justice to give a workshop. Looming on the horizon is the potential accession of Turkey, a poor country with a larger population than any country in the union. We have no way of predicting how many Turks would choose to migrate to Britain, any more than we can predict how many Romanians and Bulgarians will come here. Recall how wildly wrong the forecasts of Polish migration proved to be. But Turkish Cypriots, who due to a historical anomaly have long had rights of entry, most certainly chose to come. There are now far more Turkish Cypriots and their descendants in Britain than remain in Turkish Cyprus. However, while all northern European countries have the same income gap with countries such as Romania, Britain has distinctive attractions and so will need to negotiate special treatment. Language is a barrier to migration, but recently, English has become the lingua franca of European youth. If you were a young Romanian, would you rather go to Finland or to Britain? While the gap in GDP is usually a reasonable proxy for economic opportunities, this hasn't been the case in the prolonged European recession. For example, young Spaniards are moving to Mexico, a far poorer country, because what matters most is work. With our flexible labour market, Britain is distinctive in Europe in providing job opportunities: migrants will head this way. Finally, there is the London effect. Canada, being an immigrant society, has long learnt how to manage the process sensibly. It periodically requires migrants to settle in particular locations to avoid concentration in clusters. Britain may need to do the same. How concerned should we be that an influx from Romania and Bulgaria will be damaging for Britain? On the evidence, the short-term economic effects are likely to be trivial. Migrants themselves gain by being paid more in Britain than they would if they stayed at home, and they are the ones who capture the lion's share of any overall economic benefits. There will be letters from CEOs saying how productive migrants are likely to be. But Britain's firms have become addicted to hiring motivated migrants rather than solving the greater challenge of turning our own young people into productive workers. A former CEO of General Motors used to claim that "what's good for General Motors is good for the USA." But such sentiments are self-serving. As a recent OECD report showed, Britain has done a terrible job at skilling up our young people. Perhaps only by starving firms of willing immigrants will they face up to their social responsibilities. Paul Collier is Professor of Economics and Public Policy at the Blavatnik School of Government, Oxford University Exodus: Immigration and Multiculturalism in the 21st Century" (Allen Lane, £20) is available from the Telegraph Bookshop for £18 + £1.35 p&p.