The collapse of the Soviet Union set in train an extraordinary wave of migration, as people poured west from Eastern Europe and Central Asia, and then, fitfully, slid back.

In a report published yesterday the World Bank says that this torrent spreading over two continents in only 17 years has often been helpful, to the countries where migrants arrive and to those they left because of the sums they send home. But, it concludes, policies often fail to make the best of this potential: shutting out unskilled labour where it is needed, or driving too much into the black market.

The report, which is a large-scale and detailed attempt to measure the migration and remittances triggered by the fall of the Soviet Union, is blunt about the difficulties. Many migrants slip through countries, even continents, without showing up in official figures. The bank is necessarily tentative about its measurements and conclusions.

It estimates that there are more than three million undocumented migrants in the European Union, and perhaps 3.5 million in Russia, but, for obvious reasons, figures are elusive. Yet it is clear about the broad trends and the remarkable speed with which they have shifted.

In the first few years, the flows were triggered simply by the sudden opening of borders that had kept people from their families or their ethnic or cultural homelands.

In a decade’s time, the bank says, the motivation will be demographic, as some populations shrink (such as Germany’s and Italy’s) while others swell. Many of the countries of Eastern and Central Europe are being hit by rapid emigration and a natural decline in the ageing population left behind. The Czech Republic and Slovenia, with fast-growing economies, are among the few to boast the opposite. But at the moment the impulse to move is overwhelmingly economic, it says: people chasing more money in freer, larger markets than their own. Attempts to control large inflows of unskilled workers focus too much on border controls, it says, and too little on matching this supply with the demand.

Russia, America, Germany and Ukraine are, in order, home to the largest numbers of migrants in the world, it says (although it does not distinguish “statistical migrants” — foreign-born residents of Russia, for example, who did not move but were renamed migrants when the Soviet Union ceased to exist).

It does, however, track the huge swings even within one country. Moscow has grown from 1.5 million people at the start of “transition” to about 10.4 million. Other towns have become ghostly, with census- takers arriving to find no one left.

But many of the poorest countries have benefited hugely, it concludes, from the money sent back by migrants — more than a fifth of national income in Moldova and Bosnia-Herzegovina. It points out, however, that improvement to living standards is not automatic but depends on corruption and opportunities to invest.

Migration and Remittances: Eastern Europe and the Former Soviet Union can be found at www.worldbank.org

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