[See update below from National Review's The Corner.]
Immigration data show that for decades during the 20th century undocumented traffic between Mexico and the United States went both ways. As Ezra Klein writes for Bloomberg, "about 85 percent of new entries were offset by departures. Consequently, the growth of the undocumented population was slow."
That was then. Now is different. Over recent decades the population of illegal immigrants grew.
Doug Massey of Princeton University sociology department has a theory.
From Klein's column:
According to Massey, the rise of America’s large undocumented population is a direct result of the militarization of the border. While undocumented workers once traveled back and forth from Mexico with relative ease, after the border was garrisoned, immigrants from Mexico crossed the border and stayed.
“Migrants quite rationally responded to the increased costs and risks by minimizing the number of times they crossed the border,” Massey wrote in his 2007 paper “Understanding America’s Immigration ‘Crisis.’” “But they achieved this goal not by remaining in Mexico and abandoning their intention to migrate to the U.S., but by hunkering down and staying once they had run the gauntlet at the border and made it to their final destination.”
At The Corner, Mark Krikorian responds to this column. He writes:
... what Massey’s work really shows is the same process every developing nation in the world has experienced – a steady movement of farmers from rural areas to the cities, as agricultural production became more rationalized and modernized. It happened in Europe, it happened here, it’s happening in China, and it’s happening in Mexico. The problem is that, because of lax immigration enforcement (both at the border and the interior, including the workplace), a very large share of Mexico’s peasantry has moved to our cities instead of cities in Mexico.