WASHINGTON — Current immigrants — especially Mexicans — are less assimilated than those 100 years ago, a study to be released today found.

The study uses Census data going back more than a century to measure assimilation through various indicators such as English-learning, employment, home ownership, rates of marriage to native-born people, child bearing, naturalization, educational attainment, military service and many others.

Based on these factors, the study by the Manhattan Institute, a conservative think tank in New York, creates an assimilation index ranging from 1 to 100, with 100 being complete assimilation.

Currently, immigrants collectively have an assimilation index of 28, compared to an index of more than 50 in 1900. The study also separates economic, cultural and civic integration levels.

The study's author, however, said that the data also indicate that the pace of assimilation over the past 25 years is higher than a century ago, which means immigrants are catching up to previous levels.

"The nation's capacity to integrate new immigrants is strong," said Jacob Vigdor, an associate professor of public policy studies and economics at Duke University.

But the progress "is not present for all groups and in particular, it's not present among some of the Latin American immigrants that are at the heart of the immigration debate these days," he added.

For Mexicans, the assimilation rate is 13, according to the index. By comparison, Canadians have an assimilation index of 53 and Germans have an index of 87, the highest.

Vigdor said there could be many reasons why Mexicans have lower rates of assimilation, including that they are closer to their home country, they have more chances to speak Spanish, and they mostly come for economic reasons as opposed to immigrants who are fleeing dangerous regimes and fear going back. For example, Vietnamese immigrants had a strong incentive to accept the United States as their homeland, he said.

In addition, many Mexican immigrants are in the United States illegally and therefore are unable to meet many of the criteria for assimilation.

"If you are illegal, you can't naturalize and there are lots of jobs that you cannot get," Vigdor said.

The study also found that Mexican immigrants brought to the United States as small children have lower rates of assimilation than other groups. This is largely because of a higher level of "negative outcomes" such as teen pregnancy and incarceration, the study says.

Jeffrey Passel, a demographer with the Pew Hispanic Center, a nonpartisan research group in Washington, said that the index was flawed as a measure of assimilation.

"Assimilation is a process that takes place over time and over generations. They're not measuring that," he said.

In addition, Passel said that the snapshot of immigrant assimilation is weighed heavily toward new immigrants who are clearly going to be less integrated into the larger society. He added that the study fails to take into account that 30 percent of current immigrants are in the United States illegally, which is vastly different from a century ago.

The study also found:

• Immigrants from developed countries are not necessarily more assimilated. Immigrants born in Korea, which the World Bank classifies as a high-income country, have a lower level of assimilation than immigrants from Cuba or the Philippines, which are classified as low-income countries.

• San Diego has the highest assimilation rate among the 10 cities with the largest immigrant populations. New York City is second.