America’s estimated 11 million illegal immigrants are increasingly putting down roots across the nation. They are employed in diverse sectors of the economy and live in families that often include children who are American citizens, a new study finds.

The analysis of government data by the Pew Hispanic Center, a nonpartisan research group in Washington, D.C., found that illegal immigrants have been arriving at a rate of about 500,000 a year. Since the mid-1990s, the number of undocumented residents has exceeded the number of new legal immigrants.

In addition, there are an unprecedented 3.1 million American citizens whose parents are illegal immigrants, the author of the study, Jeffrey Passel, said yesterday. The increasing presence of mixed-status families, Mr. Passel said, “shows us this may be a population that is reluctant to leave the U.S. and return to their home country.”

The findings are particularly significant as the debate over immigration reform gains momentum on Capitol Hill, where senators soon will be faced with two competing bills that offer different ways to implement the president’s proposed guest-worker program.

Senators Kennedy and McCain proposed one solution last month for a guest-worker program that would include a path to legalization for immigrants already working illegally in America. Meanwhile, Senators Cornyn and Kyl are expected to introduce an alternate plan by the end of the month. Their plan is different, they say, in that it will be a “work and return” model, not a “work and stay” model.

The distinction is important in light of Mr. Passel’s finding that illegal immigrants are increasingly making America home. The study raises questions about the likelihood that guest workers or illegal immigrants will voluntarily leave the country, even with the opportunity to gain legal status.

In 1986, the last time lawmakers made a comprehensive effort to fix America’s illegal immigration problems, an amnesty brought about a short-lived dip in the country’s undocumented population. Congress also passed a law making it illegal for employers to hire workers lacking proof of proper immigration status, but the law was not widely enforced. In the nearly two decades since the 1986 amnesty, the Pew Center estimates the number of illegal immigrants has nearly tripled to almost 11 million from four million.

As a result, America has become increasingly dependent on illegal labor. At least 6.3 million unauthorized workers were employed as of March 2004, comprising 4.3% of the civilian labor force. In some low-skill occupations, one in four workers is undocumented, according to the study. In construction, about one in four dry-wall workers and cement masons is an illegal immigrants. The study found similar results among dishwashers and household cleaners. Farming remains the most concentrated general industry, with 19% of its workers illegal.

Almost all male illegal immigrants (92%) are employed, a significantly higher rate than American men (83%). Female undocumented immigrants, in contrast, work at lower rates (56%) than native-born American women (73%), a fact the study attributes to the high percentage who are at home raising children.

Average household income in undocumented families is just $27,400 – nearly half that of the native-born population, which is $47,700. Undocumented immigrants are often caught in lower-paying jobs and unable to move up the social ladder, the study says.

Limited job opportunities are not always due to a lack of education. While many undocumented immigrants are poorly educated, with more than half not having graduated from high school, there are also an increasing number of highly educated illegal immigrants. The study reported that 15% have college degrees and 10% have some college education. Mr. Passel said the educated comprise a growing share of the illegal population, many of whom entered legally on a work or tourist visa and overstayed it.

However, one of the most common ways that immigrants reach America is by sneaking over the Mexican border. This massive flow is a relatively recent development. Until the 1970s, there was no large-scale Mexican settlement. But in the 1980s, a combination of economic, political, and social factors began driving the mass migration that continues until today, propelling Mexicans to become America’s largest source of undocumented foreign-born residents, at 57%. At the time of the 1986 amnesty, about a quarter of Mexican immigrants entered illegally; today, 85% do, Pew estimates.

In New York, Mexican immigrants represent one of the newest and the fastest-growing major immigrant groups. New York has the fourth-largest illegal population in the country, and it is overwhelmingly composed of Mexicans who arrived during the past decade, Mr. Passel said.

New York’s illegal population continues to grow – as well as in the other core states of California, Texas, and Florida – but there has been a major redistribution toward new settlement states. Arizona, North Carolina, Georgia, and Tennessee have all seen tremendous growth, the study says.

The report was prepared for the Independent Task Force on Immigration and America’s Future, co-chaired by Spencer Abraham – a former Republican senator of Minnesota – and Lee Hamilton – who was a Democratic Congressman from Indiana.