Duke University's Jack Vigdor, who wrote the study for the libertarian Manhattan Institute, used a variety of census data to develop an assimilation index with 100 indicating full integration.

Vigdor said he found an "unprecedented" average rate of assimilation during the past quarter century due to the economic boom of the 1990s.

Demographics such as Vietnamese immigrants have an assimilation index of 41 but Mexican and Latin American immigrants have an index of 13, The Washington Post (NYSE:WPO) said Tuesday.

USA Today reports that roughly half of the 40 million immigrants in the United States come from Latin American countries and Pew Hispanic Center demographer Jeffery Passel says 30 percent of those are in the country illegally.

Vigdor says that is a major reason for the disparity among Latin American immigrants.

"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."