When the Assimilation of Immigrants Stalls THE immigration legislation percolating in the Senate has been pitched as an all-things-to-all-factions compromise. Illegal immigrants will be regularized, but most of them will have to wait at least a decade to gain citizenship. There will be more visas and new guest-worker programs, but also stiffer enforcement on the border and in workplaces. But the bill's real priority is to accelerate existing immigration trends. The enforcement mechanisms phase in gradually, with ambiguous prospects for success, while the legislation's impact on migration would be immediate: more paths to residency for foreigners, instant legal status for the 11 million here illegally, and the implicit promise to future border-crossers that some kind of amnesty always comes to those who come and wait. Today, almost 25 percent of working-age Americans are first-generation immigrants or their children. That figure is up sharply since the 1960s, and it's projected to climb to 37 percent by 2050. A vote for the Senate legislation would be a vote for that number to climb faster still. The bill has been written this way because America's leadership class, Republicans as well as Democrats, assumes that continued mass immigration is exactly what our economy needs. As America struggles to adapt to an aging population, the bill's supporters argue, immigrants offer youth, vitality and tax dollars. As we try to escape economic stagnation, mass immigration promises an extra shot of growth. Is there any reason to be skeptical of this optimistic consensus? Actually, there are two: the assimilation patterns for descendants of Hispanic (particularly Mexican) immigrants and the socioeconomic disarray among the native-born poor and working class. Conservatives have long worried that recent immigrants from Latin America would assimilate more slowly than previous new arrivals - because of their sheer numbers and shared language, and because the American economy has changed in ways that make it harder for less-educated workers to assimilate and rise. As my colleague David Leonhardt wrote recently, those fears seem unfounded if you look at second-generation Hispanics, who make clear progress - economic, educational and linguistic - relative to their immigrant parents. But there's a substantial body of literature showing that progress stalling out, especially for Mexican-Americans, between the second generation and the third. A 2002 study, for instance, reported that despite "improvements in human capital and earnings" for second-generation Mexican immigrants, the third generation still "trails the education and earnings of the average American," and shows little sign of catching up. In their 2009 book "Generations of Exclusion," the sociologists Edward Telles and Vilma Ortiz found similar stagnation and slippage for descendants of Mexican immigrants during the second half of the 20th century. As National Review's Reihan Salam points out, even a recent Pew study painting an optimistic portrait of assimilation also shows third-generation (and higher) Hispanics with lower household incomes than the second generation. This past need not predict the future. Maybe things will turn out better for the descendants of people arriving now. But it's pretty easy to see how the third-generation stall-out could continue, given the trends - unemployment, family breakdown, weakening communal ties - already working against social mobility in America. These trends mean that we're asking low-skilled immigrants to assimilate into a working class that's already in crisis. We're hoping that our dysfunctional educational system can prevent millions more children from assimilating downward into what sociologists have called a "rainbow underclass." And we're betting that the growing incomes of second-generation Hispanics will outweigh their retreat from marriage and rising out-of-wedlock birthrates. All these bets may pay off. But maybe, just maybe, we should be hedging them a bit. If we want to regularize 11 million illegal immigrants, for instance, does it make sense to layer bigger guest-worker programs on top of amnesty? If we care about workplace enforcement, why not phase it in more completely before offering legal status to the undocumented? If we want to increase immigration over all, shouldn't we consider tilting the balance much, much more toward higher-skilled immigrants than the current legislation does? On a blackboard in an economics classroom, the case for mass immigration looks airtight. But many of America's economic difficulties are rooted in social and cultural problems, and a policy that just ignores those problems is a policy that's likely to make them worse. In the end, the promise of American life is more than just a bigger paycheck than foreign economies supply. It's a promise of social equality, intergenerational advancement and fluid lines of class. The fact that so many people around the world still find that promise appealing is a wonderful thing. But it's also important to be sure, while we decide how many of them to welcome and how fast, that we can still deliver on it.