In the "age of migration," a period roughly coinciding with the years 1870-1910, massive flows of workers migrated across the globe. The growth in the labor force due to migration in the United States alone was 21 percent, while countries like Norway, Sweden, Italy, and Ireland saw their labor forces decline by 22, 18, 29 and a whopping 41 percent, respectively.

The fact that an Italian worker could earn four times as much for equivalent work in the United States as he could in Italy is thought to be a key factor explaining that mobility. Big wage-gap differentials create pressure to move.

So here's a little tidbit, derived from "Let Their People Come: Breaking the Gridlock on Global Labor Mobility" a monograph by Lant Pritchett, an economist currently working at the World Bank. Wage gap differentials between rich and poor countries today are at least twice as great as those between the U.S. and its primary sources of immigrants during the age of migration. The pressure to move is greater now than ever before.

The wage gaps in the world today are at historically high levels... it appears that the wage differentials that set in motion the mass migrations in the late nineteenth century are substantially smaller than the current gaps in real wages between potential migration partners.

If a wage gap of 4 to 1 between the United States and Italy in 1870 was sufficient to create a migration that reduced population by 30 percent over a forty-year period -- even when transport costs were higher, travel was more dangerous, and communication with loved ones left behind was much more expensive and less reliable -- then it is at least plausible that the existing wage differences indicate potential forces for substantially larger labor movements than those currently observed.

The world is extraordinarily richer today that it was a hundred years ago. But that wealth has not been evenly distributed, globally. Far from it. Income disparities between countries far overwhelm disparities within countries.

Economist Dani Rodrik tipped off How the World Works to the existence of Pritchett's monograph in his blog. I've only read the introduction and first chapter so far, but it looks like an essential source of information and ideas on how to think about the massively complex and controversial topic of labor migration, albeit from a stance that the majority of commentators on the issue will likely find politically unfeasible. I can't yet say whether any of Pritchett's policy proposals, which purport to address both the political opposition to labor migration and the inevitability of its happening, make sense. But just the initial contribution of historical perspective seems enough to chew on for now. As long as global inequities in the distribution of wealth continue to rise so to will the pressure on people to move around the globe. Securing borders against that flow doesn't seem like a very good long term bet.