SAN FRANCISCO (Reuters) - 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.