Allowing just 5 percent of people from poor countries to emigrate to wealthier countries may increase global GDP by several trillion dollars a year. That's the conclusion of a paper published in the Summer 2011 issue of the Journal of Economic Perspectives, by Michael Clemens, an economist at the Center for Global Development and a visiting scholar at New York University, who surveys research on global mobility and finds that the gains from a 5 percent increase in immigration would exceed the gains from eliminating all government-imposed barriers to the global flow of goods and capital.
One reason a little more immigration adds so much value is that people can become much more productive simply by changing locales. In previous work, Clemens estimated that a construction worker in Accra, Ghana, could increase his productivity by 700 percent to 1,000 percent just by moving to a job site in any major U.S. city.
Last year the U.S. Diversity Visa Lottery, which awards immigration slots mainly to people from developing countries, received 13.6 million applications for 50,000 slots.