SAN FRANCISCO - Immigrants are far less likely than the average U.S.-born citizen to commit crime in California, the most populous state in the United States, according to a report issued late on Monday.

People born outside the United States make up about 35 percent of California's adult population but account for about 17 percent of the adult prison population, the report by the Public Policy Institute of California showed.

According to the report's authors the findings suggest that long-standing fears of immigration as a threat to public safety are unjustified. The report also noted that U.S.-born adult men are incarcerated at a rate more than 2 1/2 times greater than that of foreign-born men.

"Our research indicates that limiting immigration, requiring higher educational levels to obtain visas, or spending more money to increase penalties against criminal immigrants will have little impact on public safety," said Kristin Butcher, co-author of the report and associate professor of economics at Wellesley College.

The study did not differentiate between documented immigrants and illegal immigrants.

The question of what to do about the millions of undocumented workers living in the United States has been one of the major issues in the U.S. presidential election. Mexico, which accounts for a high proportion of illegal immigrants in California, was deeply disappointed at the U.S. Congress' failure to pass President George W. Bush's overhaul of immigration laws last year.

When Butcher and her co-author, Anne Morrison Piehl, associate professor of economics at Rutgers University, considered all those committed to institutions including prison, jails, halfway houses and the like, they found an even greater disparity.

Among men 18 to 40, the population most likely to be in institutions because of criminal activity, the report found that in California, U.S.-born men were institutionalized 10 times more often than foreign-born men (4.2 percent vs. 0.42 percent).

Among other findings in the report, non-citizen men from Mexico 18 to 40 -- a group disproportionately likely to have entered the United States illegally -- are more than eight times less likely than U.S.-born men in the same age group to be in a correctional institution (0.48 percent vs. 4.2 percent).

"From a public safety standpoint, there would be little reason to further limit immigration, to favor entry by high-skilled immigrants, or to increase penalties against criminal immigrants," the report said.

Reporting by Duncan Martell; Editing by Adam Tanner and Bill Trott
© 2008 Reuters