In 2008, Clemens and his frequent co-writer, Harvard economist Lant Pritchett, came up with a new statistic called "income per natural." Their goal was to show "the mean annual income of persons born in a given country, regardless of where that person now resides." They found that large percentages of people from Haiti, Mexico, and India who live above international poverty lines don't actually reside in their home countries. "For example, among Haitians who live either in the United States or Haiti and live on more than $10 per day--about a third of the U.S. 'poverty' line--four out of five live in the United States," Clemens wrote. "Emigration from Haiti, as a force for Haitians' poverty reduction, may be at least as important as any economic change that has occurred within Haiti."

The trillions of dollars are lost by not maximizing human potential. Workers in the developing world can be much more productive when they are not locked in places with crumbled infrastructure, poor academic institutions, and mass corruption. "It's the biggest arbitrage opportunity in the world," Clemens told me. "It's hard to find a cell phone or pair of jeans that sells for a thousand percent price difference in two different countries, and yet the labor of a McDonald's worker, the labor of a child care worker, the labor of a construction worker, does sell for thousand percent differences between Haiti and the United States."

If wealthy nations open their borders, won't native workers lose their jobs or see their pay shrink? Not so, according to Clemens. He and his co-authors, through study of all the available economic literature, have found that decades of immigration of tens of millions of people to the United States has reduced real wages for the average American worker by fractions of a percent, if at all. Meanwhile, immigrants to the U.S. from developing countries can increase their income by 100 percent, or 1,000 percent. "Immigration is very, very far from being a zero-sum game of 'their poverty or ours,'" Clemens wrote in 2010. "Within ranges that even slightly resemble current migration levels, it is rather simply 'their poverty or their prosperity,' while we remain prosperous."

Clemens's research also challenges the notion that immigrants take away jobs from Americans. In agriculture, for example, he has estimated that for every three seasonal workers who are brought in, one American job is created across all sectors. Directly, workers need managers, and more often than not those managers are Americans. Indirectly, workers buy things, which means more Americans are needed to sell and produce those things. And yet, Clemens told me, "when a bus of 60 Mexicans is coming up from the border, nobody looks at it and says 'Ah, there's 20 American jobs.'"

But some immigration restrictionists have far bigger worries than workers losing small percentages of their salaries. There are many possible negative consequences of open borders. Naik points out that "political externalities" may be a major drawback of allowing anyone who wants to move to stable, wealthy nations to do so. Gallup polls have found that 700 million people would like to permanently move to another country, many of them from developing nations with failed political systems. If the U.S. or another wealthy nation were to see a sudden large increase in immigrants from these countries, it's possible that the new populace will vote for bad policies in their new home. As Naik puts it, some people believe that "if you're coming from a place that has a problem, you are probably part of the problem, and if you move to a new place you might bring the problem with you."