Immigration is an especially trying issue because the conflict is not only among all of us but inside each of us. The most overt racist must still wonder who would clean up his mess if every immigrant disappeared. The most mushy-hearted liberal must still worry about the effect on America's working (or would-be working) poor of having a reserve army of immigrants perched along our border with Mexico.

A common dodge is to say, "I'm not against immigration. I'm just against illegal immigration. We have the sovereign right to control our borders." But people who say this generally favor a crackdown on illegals without any compensating increase in the quotas for legal immigrants. So it is immigration that they oppose, not just illegal immigration.

The only mainstream voice with a logically consistent position is the Wall Street Journal, which says: open the floodgates. Immigrants made America great and still can. Let them all in. As a grandson of immigrants (how about you, dear reader?), I want badly to agree. But I just don't have the guts to dismiss every concern of people who say there must be a limit. So OK, there must be a limit. But we haven't hit it yet.

The number of illegal immigrants actually has been declining. The Department of Homeland Security (how the heck did they get into this?) reported a drop from 11.6 million in 2008 to to 10.8 million in 2009. The reason is not a crackdown at the border. The overwhelming reason is the same recession that has stirred up anti-immigrant alarm. The more the American economy declines, the fewer people want in on it.

Decline is not a good solution to the illegal immigrant problem, obviously. But this does illustrate an important point, which is that immigration--legal or illegal--is primarily an economic decision. And like any economic decision, it is made at the margin. At any given time, most Mexicans have decided not to emigrate. The ones who try to cross the border are those for whom things are just bad enough and the promise of America is just great enough to justify the cost and the risk.

What this suggests is that if you're willing to settle for controlling illegal immigration, and not the futile goal of eliminating it, you ought to consider a larger role for helping the Mexican economy in your policy mix. It is a very small part of President Obama's proposed reforms, and Mexico is actually too rich already to qualify for traditional aid programs (though the distribution of income is terrible).

But isn't this hopeless? The economic gap between Mexico and the United States is huge and will never close. People will always prefer to live and work in the United States. Answer: the gap doesn't have to close. It just has to get smaller. (Because of faster growth in Mexico, we hope, rather than slower growth in the United States.) Every time the gap gets smaller, there will be Mexicans who decide--entirely on their own, without the help of the Arizona police--that they are better off staying where they are.

Further Reading
Tom Friedman had a good piece on Mexico the other day. And here is a PDF of the IMF’s latest annual report on the Mexican economy. President Obama’s immigration reform plan is here .