Immigrants of the past quarter-century have been assimilating in the United States at a notably faster rate than did previous generations, according to a study released today.

Modern-day immigrants arrive with substantially lower levels of English ability and earning power than those who entered during the last great immigration wave at the turn of the 20th century. The gap between today's foreign-born and native populations remains far wider than it was in the early 1900s and is particularly large in the case of Mexican immigrants, the report said.

The study, sponsored by the Manhattan Institute, a New York think tank, used census and other data to devise an assimilation index to measure the degree of similarity between the United States' foreign-born and native-born populations. These included civic factors, such as rates of U.S. citizenship and service in the military; economic factors, such as earnings and rates of homeownership; and cultural factors, such as English ability and degree of intermarriage with U.S. citizens. The higher the number on a 100-point index, the more an immigrant resembled a U.S. citizen.

In general, the longer an immigrant lives in the United States, the more characteristics of native citizens he or she tends to take on, said Jacob L. Vigdor, a professor at Duke University and author of the study. During periods of intense immigration, such as from 1870 to 1920, or during the immigration wave that began in the 1970s, new arrivals tend to drag down the average assimilation index of the foreign-born population as a whole.

The report found, however, that the speed with which new arrivals take on native-born traits has increased since the 1990s. As a result, even though the foreign population doubled during that period, the newcomers did not drive down the overall assimilation index of the foreign-born population. Instead, it held relatively steady from 1990 to 2006.

"This is something unprecedented in U.S. history," Vigdor said. "It shows that the nation's capacity to assimilate new immigrants is strong."

A major reason for these disparities in assimilation levels may be the high percentage of Mexican immigrants who are in the country illegally, Vigdor said. When only cultural factors are considered, Mexicans score almost as high as Vietnamese and higher than immigrants from countries such as India and China, which tend to have a high rate of immigration to the United States.

"If you're in the country illegally, a lot of the avenues of assimilation are cut off to you," he said. "There are lot of jobs you can't get, and you can't become a citizen."