The report said prison growth and higher incarceration rates do not reflect an increase in the nation's overall population. Instead, it said, more people are behind bars mainly because of tough sentencing measures, such as "three-strikes" laws, that result in longer prison stays.

"For some groups, the incarceration numbers are especially startling," the report said. "While one in 30 men between the ages of 20 and 34 is behind bars, for black males in that age group the figure is one in nine."

The racial disparity for women also is stark. One of every 355 white women aged 35 to 39 is behind bars, compared with one of every 100 black women in that age group.

The nationwide figures, as of Jan. 1, include 1,596,127 people in state and federal prisons and 723,131 in local jails. That's out of almost 230 million American adults.

The report said the United States incarcerates more people than any other nation, far ahead of more populous China with 1.5 million people behind bars. It said the U.S. also is the leader in inmates per capita (750 per 100,000 people), ahead of Russia (628 per 100,000) and other former Soviet bloc nations which round out the Top 10.

The U.S. also is among the world leaders in capital punishment. According to Amnesty International, its 53 executions in 2006 were exceeded only by China, Iran, Pakistan, Iraq and Sudan.