Because of our long land border with Mexico (the Rio Grande is a trickle most of the year), it has been far easier to emigrate illegally from Mexico than from any other country.
As a result, Mexican immigrants tend to be younger, poorer, less educated and less fluent in English than immigrants from other countries. They are also more likely to be illegal -- Mexicans are 30 percent of all immigrants but 58 percent of illegals -- and less likely to become U.S. citizens.
A continued standstill in Mexican immigration means that the number of illegals in the United States will probably continue to decline, even in an economic recovery. Children of illegals born in the U.S., who are automatically U.S. citizens, don't add to the illegal numbers.
And no other country has produced or is likely to produce anything close to the number or share of illegals.
The central focus of the debate over the so-called comprehensive immigration bills that came to the floor of the Senate in 2006 and 2007 was their provisions for legalization of those illegally here -- amnesty, to opponents. On the campaign trail, Barack Obama is promising to push for such legislation just as he promised in 2008.
But he didn't deliver when Democrats had supermajorities in both houses and is unlikely to get anywhere on this project in a second term.
It may not matter much. With the Mexican reservoir of potential illegals dried up, and with better border enforcement and increased use of the much improved e-Verify system in workplaces, the illegal population seems likely to decline.
The key immigration issue for the future is whether America, like our Anglosphere cousins Canada and Australia, will let in more high-skill immigrants.