How mobile are the world''s top research scientists, and do they migrate disproportionately to the richest countries? Writing in the June 2009 Economic Journal, Andrew Oswald and colleagues analyse data on the migration and productivity of Nobel Prize winners and the world''s most highly cited physicists.

There is a large research literature on the ''brain drain'', but few researchers have looked at migration among world-class scientists. In a recent study, we analysed the international movement and productivity levels of elite research scientists, including Nobel Prize winners and a data set, which we constructed, of 158 of the world''s most highly cited physicists. We draw five conclusions.

First, the UK currently wins fewer Nobel Prizes in science than it used to, and the&##160;United States garners many more. What is less widely known is that in both the UK and the United States, immigrant scientists win the Nobel Prize less often, proportionately,&##160;than in earlier decades.

Second, by charting the careers of a group of distinguished physicists, we show that they are strikingly mobile: nearly half of the world''s most highly cited physicists work outside their country of birth. Our 158 physicists were born in 32 countries but now live in only 16. Approximately 30% migrated after their first degrees, and they went predominantly to the United States (see Figure 1).

Third, among highly cited physicists, the average productivity (as measured by a citations index called the h-index) of movers is not different from that of stayers. We are unable, with our data, to say whether migration itself causally increases a scholar''s productivity, but it might be argued that there are no strong externality effects among senior scholars.

Fourth, international flows of physicists between their first degree and the present day demonstrate that top scholars head to countries with high levels of R&D spending. Switzerland and the United States are the world''s large importers, per capita, of elite physicists. CERN, the particle physics laboratory in Switzerland, must play some role here, but because of difficulties caused by multiple affiliations, we have not attempted to separate out those scientists.

Fifth, we find evidence that among elite physicists, a current affiliation in the United States is associated with a 13-19% higher h-index. This may be a genuine productivity difference or reflect some form of pro-US citations bias or some mixture of the two.

How, conceptually, can we make sense of the data? One way to view the findings on physicists is as supporting a theoretical model in which in the modern globalised world, the costs of migration are low. Intuitively, the idea is the following.

Consider a world with very high costs – whether because of cultural differences across societies or costly travel or poor communication – of switching between countries. Then only the very best workers will migrate. This is because they alone are the ones who will make a big enough return from international labour mobility to outweigh the high costs. In this case, migrants will be disproportionately from the top end of the ability distribution. They will be outstanding scientists with, in our terminology, particularly large h-indexes.

Now contrast this with the case of low mobility costs. In that case, elite scientists of more average kinds of abilities, like the norm within the country into which they migrate, will find it rational to choose to switch countries. Hence mobile incoming scientists will be of similar quality to the average of those in the receiving country.

Most of these newcomers will not go on to win science prizes in the way that happened in an older world – think of an early twentieth century setting of ocean liners and telegrams – where mobility costs were high. Any increases through time in the wage premium earned by distinguished scientists in the rich receiving countries will act to reinforce these tendencies.

This article summarises ''The Elite Brain Drain'' by Rosalind Hunter, Andrew Oswald and&##160;Bruce Charlton, Economic Journal 119(538): F231-51 (February 2009).