It’s hardly news that, like Arizona, many states and scores of cities have been looking for ways to drive illegal immigrants out – ordering cops to detain people who can’t show documents verifying their right to be here, passing measures to fine landlords who rent to illegal immigrants and employers who hire them. And there are the Republicans who want to fiddle with the Constitution to deny children born to illegal aliens birthright citizenship.

   So obviously Americans must be panicking because illegal immigration has been spiking, right?

   Wrong. A new report from the Pew Hispanic Center, like a similar Census report earlier this year, indicates a dramatic decline not only in the number of people entering the country illegally between 2007 and 2009 — for Mexicans, it’s down by 70 percent from three years ago — but in the number of illegal aliens living here, down from roughly 12 million in 2007 to 11.1 million today.

   It’s not a new paradox, if it’s a paradox at all. Nobody can be sure how much of the decline in illegal immigration and in the number of resident illegal aliens in recent years is due to more stringent enforcement, as anti-immigration groups like the Center for Immigration Studies claim, and how much is the recession.

   And since recessions and unemployment inevitably feed the backlash against immigration, it becomes even more difficult to tease them apart. The Pew report fastidiously avoids any making choice between them. Almost certainly it’s some of each.

   But if you look at the economic cycles of the past, especially in the years when there was no federal enforcement – before there were “illegal” immigrants, before the Border Patrol, the Immigration and Naturalization Service, before ICE – there was a consistent link between the U.S. economy, events abroad (the Irish potato famine, for example; or the Russian pogroms) and the flow of aliens to this country.

   The Depression of the 1870s drove annual immigration down from 460,000 in 1873 to 178,000 in 1879. Similarly, the severe Depression of the 1890s drove the annual number of arrivals from an average of more than 500,000 in the 1880s to less than 270,000 a year in the middle of the not-so-gay nineties and to 229,000 in 1898. With recovery after the turn of the century immigration climbed to historic highs, sometimes running to over one million annually in the decade before World War I.  

   To oversimplify only a bit, immigration has always been a trailing indicator of the U.S. economy. When times were good, they came; when times turned bad, they not only came in much smaller numbers, but many went back. Sometimes the migrations took a few months to slow; sometimes it took a year. Given modern communications and the speed of travel, the contemporary effects come faster, but they probably come just as surely.

   The new numbers from Pew also reconfirm earlier data indicating the great shift in the destinations of new immigrants. Where the greatest flow had once gone to California and the Southwest, it was now going to the packing plants of the Midwest and the farms of the Southeast. Nonetheless, California still has a larger share of the immigrant labor force other than Nevada. California is also home to 23 percent of illegal aliens, far and way the largest percentage of any state. 

   Because border enforcement had been concentrated on the historic routes into Texas and California, it pushed much more of the traffic into Arizona. Inevitably, the backlash followed, especially in places that had never seen Latinos before.

   Yet, notwithstanding the declines of the past three years, the numbers of aliens living in this country without documentation still remains far higher than it was a decade ago and triple what it was in 1990.

   That increase, in the view of scholars, is the really ironic unintended consequence of upgraded enforcement at the border. Because they made the border crossing ever more expensive and dangerous, the beefed-up Border Patrol, now with four times the manpower it had twenty years ago, the walls and fences, the sensors and other electronics changed the historic migration pattern.

   Millions who once shuttled seasonally between jobs in the U.S. and their homes in Mexico or further south simply stayed put, sent for their families and became permanent, if sometimes reluctant, residents of the United States. The immigrants come when the economy needs them. They leave when it doesn’t. 

   But the most significant thing the Pew numbers seem to show – and what California seems to prove – is that it’s not absolute numbers that drive anti-immigrant backlash, but the pace of immigration – and the degree of familiarity with the ethnic diversity that immigration brings.

   California had its Arizona episode in the first half of the 1990s, when we passed Proposition 187, the initiative that sought to deny all public services, schooling included, to illegal aliens. It’s unlikely that California will have such a period again. Nor, in the long run, will the nation. In another thirty or forty years, the United States will no longer have a non-Hispanic white majority. Well before then, our immigration politics – and our policy – are likely to be radically different. .

   Already most younger Americans seem far more comfortable than their elders with our growing diversity, just as they are more comfortable with gay marriage and other social issues that drive some of their parents to the wall, or maybe the Tea Party. Nothing is forever.