WASHINGTON -- Big increases in immigration since 1990 have not hurt employment prospects for American workers, says a study released Thursday.

The report comes as Congress and much of the nation are debating immigration policy, a big issue in this fall's midterm congressional elections.

The Pew Hispanic Center found no evidence that increases in immigration led to higher unemployment among Americans, said Rakesh Kochhar, who authored the study.

Kochhar said other factors, such as economic growth, played a larger role than immigration in determining the job market for Americans.

The study, however, did not look at whether wages were affected by immigration. Advocates for tighter immigration policies argue that immigrant workers depress wages for American workers, especially those with few skills and little education.

Immigration supporters argue that foreign workers often take jobs that Americans don't want and won't take.

The Pew Hispanic Center is a non-partisan research organization that does not advocate policy positions. The center studied census data on the increase in immigrants from 1990 to 2000, and from 2000 to 2004, for each state. It matched those figures with state employment rates, unemployment rates and participation in the labor force among native-born Americans.

The U.S. had 28 million immigrants -- legal and illegal -- age 16 and older in 2000, an increase of 61 percent from 1990. By 2004, there were 32 million.

Among the study's findings:

• Twenty-two states had immigration levels above the national average from 1990 to 2000. Among them, 14 had employment rates for native-born workers above the national average in 2000, and eight had employment rates below the national average.

• Twenty-eight states and the District of Columbia had immigration levels below the national average from 1990 to 2000. Among them, 16 had above-average employment rates for native-born workers in 2000, and 13 had below-average employment rates.

• Twenty-four states had immigration levels above the national average from 2000 to 2004. Among them, 13 states had employment rates for native-born Americans above the national average in 2004, and 11 had employment rates below the national average.

• Twenty-six states and the District of Columbia had immigration levels below the national average from 2000 to 2004. Among them, 12 had employment rates for native-born Americans above the national average, and 15 had employment rates below the national average.

Immigrants tend to be younger and have less education than American workers. The study, however, found "no apparent relationship between the growth of foreign workers with less education and the employment outcome of native workers with the same low level of education."

However, Steven Camarota, director of research for the Center for Immigration Studies, said his research shows that many young workers with little education are hurt by competition from immigrants.

"Employment for less educated natives has declined, and their wages have declined," said Camarota, who advocates stricter immigration policies. "There is no shortage of less educated workers in the United States."

Copyright 2006 The Associated Press