The economists who agree with Card also make an intuitive point, inevitably colored by their own experience. To the Israeli-born economist whose father lived through the Holocaust or the Italian who marvels at America's ability to integrate workers from around the world, America's diversity -- its knack for synthesizing newly arrived parts into a more vibrant whole -- is a secret of its strength. To which Borjas, who sees a different synthesis at work, replies that, unlike his colleagues, the people arriving from Oaxaca, Mexico, are unlikely to ascend to a university faculty. Most of them did not finish high school. "The trouble with the stories that American journalists write about immigration," he told me, "is they all start with a story about a poor mother whose son grows up to become.. . ." and his voice trailed off as if to suggest that whatever the particular story -- that of a C.E.O., a ballplayer or even a story like his own -- it would not prove anything about immigration. What economists aim for is to get beneath the anecdotes. Is immigration still the engine of prosperity that the history textbooks describe? Or is it a boon to business that is destroying the livelihoods of the poorest workers -- people already disadvantaged by such postmodern trends as globalization, the decline of unions and the computer?

$( The Lopsided-Skill-Mix Problem $)

This spring, while militias on the prowl for illegal immigrants were converging on the Arizona border and, on the other side of the political fence, immigrant protesters were taking to the streets, I sampled the academic literature and spent some time with Borjas and Card and various of their colleagues. I did not expect concurrence, but I hoped to isolate what we know about the economic effects of immigration from what is mere conjecture. The first gleaning from the Ivory Tower came as a surprise. All things being equal, more foreigners and indeed more people of any stripe do not mean either lower wages or higher unemployment. If they did, every time a baby was born, every time a newly minted graduate entered the work force, it would be bad news for the labor market. But it isn't. Those babies eat baby food; those graduates drive automobiles.

As Card likes to say, "The demand curve also shifts out." It's jargon, but it's profound. New workers add to the supply of labor, but since they consume products and services, they add to the demand for it as well. "Just because Los Angeles is bigger than Bakersfield doesn't mean L.A. has more unemployed than Bakersfield," Card observes.

In theory, if you added 10 percent to the population -- or even doubled it -- nothing about the labor market would change. Of course, it would take a little while for the economy to adjust. People would have to invest money and start some new businesses to hire all those newcomers. The point is, they would do it. Somebody would realize that the immigrants needed to eat and would open a restaurant; someone else would think to build them housing. Pretty soon there would be new jobs available in kitchens and on construction sites. And that has been going on since the first boat docked at Ellis Island.

But there's a catch. Individual native workers are less likely to be affected if the immigrants resemble the society they are joining -- not physically but in the same mix of skills and educational backgrounds. For instance, if every immigrant were a doctor, the theory is, it would be bad for doctors already here. Or as Borjas asked pointedly of me, what if the U.S. created a special visa just for magazine writers? All those foreign-born writers would eat more meals, sure, but (once they mastered English, anyway), they would be supplying only one type of service -- my type. Bye-bye fancy assignments.

During the previous immigrant wave, roughly from 1880 to 1921 (it ended when the U.S. established restrictive quotas based on country of origin), the immigrants looked pretty much like the America into which they were assimilating. At the beginning of the 20th century, 9 of 10 American adults did not have high-school diplomas, nor did the vast majority of immigrants. Those Poles and Greeks and Italians made the country more populous, but they did not much change the makeup of the labor market.

This time it's different. The proportion of foreign-born, at 12 percent, remains below the peak of 15 percent recorded in 1890. But compared with the work force of today, however, the skill mix of immigrants is lopsided. About the same proportion have college degrees (though a higher proportion of immigrants are post-graduates). But many more -- including most of the those who have furtively slipped across the Mexican border -- don't have high-school diplomas.