The United States is a country that has been peopled largely by vast surges of migration -- from the British Isles in the 18th century, from Ireland and Germany in the 19th century, from Eastern and Southern Europe in the early 20th century, and from Latin America and Asia in the last three decades.
Going back in history, almost no one predicted that these surges of migration would begin -- and almost no one predicted that they would stop when they did.
Thus when the 1965 Immigration Reform Act was passed, almost no one predicted that we would have massive immigration from Mexico. Experts told us that immigrants came in large numbers only from Europe.
The experts got that wrong. From 1980 to 2008, more than 5 million Mexicans legally entered the United States. And Mexicans account for about 60 percent of the estimated 11 million illegal immigrants in the U.S. today.
Immigration policymakers have assumed that the flow of Mexican immigrants would continue indefinitely at this high level. But now evidence is accumulating that this vast surge of migration is ending.
The Pew Hispanic Center, analyzing Census statistics, has estimated that illegal Mexican entrants have been reduced from 525,000 annually in the 2000-04 years to 100,000 in 2010.
"The flow has already stopped," Douglas Massey of the Mexican Migration Project at Princeton recently told The New York Times. "The net traffic has gone to zero and is probably a little bit negative."
One reason is the deep recession and slow economic recovery here in the United States. Tens of thousands of construction jobs, once plentiful in high-immigration states, have disappeared. Foreclosures on mortgages that should never have been granted have been especially high among Hispanics.
State laws, like Arizona's law requiring use of the federal e-Verify system to check on immigration status of new hires, have clearly had some impact. And the cost of crossing the border illegally has sharply increased.
The Pew Hispanic Center estimates the 2010 illegal population at 11.2 million, down from the 2007 peak of 12.0 million and just about the same level as in 2005. It's probably lower today.
Even more important, things have changed in Mexico. Its birth rate has fallen from 7 children per woman in 1971 to 3.2 in 1990 and 2 in 2010, barely enough to prevent population loss.
Mexico has finally become a majority middle-class country, former Foreign Minister Jorge Castaneda argues in his recent book "Manana Forever?" Mexico has more cars and television sets than households now, most Mexicans have credit cards, and there are almost as many cell phones as people.
There has been a boom in higher education, especially in technical schools. The increasing numbers of well-educated Mexicans have no need to go to the United States to live a comfortable and even affluent life. Mexico has grown its way out of poverty.
The historic experience has been that countries cease generating large numbers of immigrants when they reach a certain economic level, as Germany did in the 1880s. Mass migration from Puerto Rico, whose residents are U.S. citizens, ended in the early 1960s, when income levels reached one-third of those on the mainland.
All of which has implications for U.S. immigration policy. It seems clear that tougher enforcement measures, like requiring use of e-Verify, can reduce the number of illegals in the United States. Returning to Mexico is a more attractive alternative than it used to be.
And the desire of legal immigrants to bring in collateral relatives under family reunification provisions is likely to diminish. That means we can shift our immigration quotas to higher-skill immigrants, as recommended by a panel convened by the Brookings Institution and Duke University's Kenan Institute and as done currently by Canada and Australia.
Such a change would be in line with the new situation. Mexican immigrants have tended to be less educated and lower-skill than immigrants from other Latin or Asian countries. Lower Mexican immigration means lower low-skill immigration. Employers of such immigrants may have to adjust their business models.
Probably they are already doing so. But government adjusts more slowly.
Barack Obama has been calling for immigration legislation similar to what George W. Bush sought, legislation geared to a status quo that no longer exists and seems unlikely to return. That's going nowhere. But sooner or later we should adjust the law to address the new emerging reality.