It’s important to keep in mind that low-skilled workers also make up the bulk of illegal immigrants. A visa limit of 20,000 a year, or even 200,000, doesn’t begin to match the realistic cross-border flow.

According to the Mexican Migration Monitor run by Roberto Suro of the Annenberg School, illegal Mexican migration exceeded 368,000 in every year from 2003-2009, with a high of 682,000 in 2007. The number has fallen with the slow U.S. recovery (to 165,000 in 2011), but history shows it will expand again if we ever get a decent job market.

The point of all this for the reform debate is that these migrants come to the U.S. to work, and they will keep coming illegally if a guest-worker program is too small or too cumbersome to use.