In the heart of California's iconic Orange County—home to Disneyland and the bourgeois teens of MTV's Laguna Beach—is troubled Santa Ana. The county seat of 353,000, where nearly 6out of every 10 adults over age 25 lack a high school diploma, suffers from crippling poverty and an explosion in crime. In 2004, the Nelson A. Rockefeller Institute of Government placed Santa Ana at the very top of its Urban Hardship Index—officially dubbing it worse off than Miami, Detroit, Cleveland, and Newark, N.J. With 76 percent of its population Hispanic, mostly Mexican immigrants, Santa Ana is the poster child for the troubles of the country's immigration policies and of Mexican immigrants in particular.

Now, a new study lays bare what sociologists and others have long argued: Mexican immigrants are assimilating to life in the United States less successfully than other immigrants. Sponsored by the conservative Manhattan Institute think tank, "Measuring Immigrant Assimilation in the United States" by Jacob Vigdor, a professor of public policy studies and economics at Duke University, introduces a novel assimilation index that uses census and other survey data to measure how similar select immigrant groups are to native-born Americans. Using such factors as intermarriage, English ability, military service, homeownership, citizenship, and earnings, Vigdor assembled a 100-point assimilation index. The closer to 100, the more assimilated an immigrant group. Overall, the report shows immigrants are weaving into the American fabric at a remarkable clip, despite arriving poorer and knowing less English than immigrants of a century ago. And they are gaining speed, with new arrivals assimilating faster than those who came more than 20 years ago. With a score of 53, Canadians are the most assimilated, followed closely by Filipinos, Cubans, and Vietnamese. The main outlier: Mexicans, with a score of 13—followed by Salvadorans.

Why Mexicans are faring so poorly in the United States is complicated, experts say. But the root of the problem is no surprise: Many Mexicans are here illegally, depriving them of rungs on the economic ladder and the opportunity to gain citizenship. "There are certain jobs or certain services you just can't get [as an illegal immigrant]," Vigdor says. "There are plenty of indications here that for those Mexican immigrants who are interested in making a more permanent attachment to the United States, their legal status puts very severe barriers in that path."

Since the 1990s, Mexicans' immigrant story has differed from that of their peers. When comparing Mexicans and Asians, "Asians show up with a lot more money, oftentimes," notes Dowell Myers, a demographer at the University of Southern California. "They have a higher education to begin with, and many of them are entrepreneurs." Past decades saw influxes of refugees from countries such as Vietnam and the Philippines. Today's Asian immigrants are some of the best and brightest, which puts them on a faster track to assimilation via economic success.

The Asian experience recalls a general rule of today's immigrants. The farther you have to migrate, the wealthier you probably were in your country of origin. "Poor people can't afford a plane trip across the ocean, but poor people can walk across the border," Myers says. "Poor Africans and poor Chinese can't do it." Because of their proximity to the United States, poor Mexicans can make the trip. Indeed, their poverty impels them to risk the border crossing. But when they arrive, they arrive significantly disadvantaged, and they often qualify for jobs that offer little opportunity for social advancement. Other factors may also contribute but are more difficult to quantify: The leading contender is that the sheer number of Latinos in the United States has created a subculture that slows assimilation.

Indeed, in a unique multigenerational study spanning four decades, Generations of Exclusion, sociologists Edward Telles and Vilma Ortiz found that many immigrants and their children had made slow progress assimilating for cultural and economic reasons. A large community means a large dating pool: Only 17 percent of third-generation Mexicans studied had married non-Hispanics. The authors found adult Mexican-Americans in the third and fourth generations lived in more segregated neighborhoods than they did as youths, largely because of the many new immigrant arrivals. Educational levels, meanwhile, lagged behind the national average. However, English ability was nearly universal, even among first-generation immigrants, which should ease the concerns of some lawmakers who want to make English the natural language. Significantly, though, 36 percent of fourth-generation Mexican-Americans studied could still speak Spanish.

Perhaps most telling: Of the approximately 1,500 surveyed in two distinct immigrant communities—Los Angeles and San Antonio—most identified as "Mexican" or "Mexican-American" even into the fourth generation. It's that kind of cultural signifier that has so many white Americans concerned that this is a group not interested in becoming American.

Ortiz says her interviews demonstrated that that was not the case. She argues that the above factors, especially segregated neighborhoods, "all probably lead to a stronger sense of identification of being Mexican or Mexican-American," she says. "The fact that they are maintaining a sense of Mexicanness is to some extent a reaction to how American society doesn't fully accept Mexican-origin folks." The continued ethnic identification is similar to that of other groups that have felt oppression from the majority, African-Americans and American Indians among them. Rubén Rumbaut, a professor of sociology at the University of California-Irvine, notes that "the people who have been most ostracized, stigmatized, and racialized...assert that now with pride, and they dig in their heels, and they become that which they had been labeled pejoratively."

Rumbaut, a leading researcher in immigrant studies, argues that assimilation is like a tango. Each party has to avoid stepping on the other's toes. "Assimilation, unlike acculturation...includes how they are welcomed or not by native groups," he says. In one study that included members of 77 nationalities, Rumbaut asked participants if they agreed that the United States was the best country in the world. Those most likely to agree were Vietnamese and Cubans or those who had benefited from refugee assistance. The least likely were Haitians, Jamaicans, and others with black skin "who bore the brunt of racial discrimination in their adoptive society," Rumbaut says. "The moral of the story is you reap what you sow. I see assimilation as a relationship and not some robotlike process of adaptation to a new environment that takes place only on the part of the assimilated."

The fact is that despite all this, Mexican immigrants and their children are advancing. That's because just about every immigrant, no matter what the country of origin, is here to work. "Couch potatoes don't emigrate," Rumbaut says. Indeed, Mexican immigrants start with nearly nothing "but actually climb more than Asians do," Myers points out. "The Mexican immigrants are the poorest of immigrants, by and large, but the majority become homeowners in the United States." That includes the folks here illegally. Myers calls that a far better measure of assimilation than self-identifiers such as "Mexican" or Mexican-American." He likens that to New Yorkers in Los Angeles rooting for the Yankees. "They wave the Mexican flags...but these are the same people who will enlist in the U.S. Army and be proud about it. Their identity is a composite of their heritage and current loyalty."