Immigrants are assimilating into American culture about as quickly as they did in 1990, reports the Manhattan Institute for Policy Research.

That is so even though their numbers have doubled since then.

The institute studied three areas to measure how well newcomers fit in — economic, cultural and civic.

The study used census data from 1890 to 2006 to help determine the assimilation rate of recent immigrants as compared to past trends.

One discovery was that Mexicans, the largest immigrant segment, are not fitting in with native-born Americans as well as other groups.

Jacob Vigdor, a Duke University professor of economics who created the index, said: "Mexicans and other Latin Americans are assimilating slowly. On the other hand, many immigrant groups are doing quite well and assimilating quite rapidly."

Of the 40 million immigrants in the United States, about half are Latin American. As USA Today points out, part of the controversy surrounding immigration is the issue of how well new immigrants fit in.

Immigrants today assimilate much faster than those of a century ago, Mr. Vigdor said.

Some of the measurements used were economic (employment, occupations, education, home ownership); cultural (ability to speak English, marriage to natives, number of children), and civic (naturalization, military service).

In response to the slowness of Mexican assimilation, Jeffrey Passel, demographer at the Pew Hispanic Center, raised an important point: "The long-term question is how do the children and grandchildren of immigrants do? Those are the groups who really do the integrating."

Mr. Passel also said that 30 percent of all immigrants in the United States are here illegally.

Such studies help us understand population trends in America as well as who we are as a people. They also assist in further defining the immigration picture and inform the debate.