The United States imprisons more people than any other country in the world, according to a report.
The report by the Pew Center on the States said the American penal system held more than 2.3 million adults at the start of the year.
The far more populous nation of China ranked second with 1.5 million behind bars, with Russia a distant third with 890,000 inmates.
"Beyond the sheer number of inmates, America also is the global leader in the rate at which it incarcerates its citizenry, outpacing nations like South Africa and Iran," according to the report.
For the first time in the nation's history, more than one in every 100 American adults is confined in a prison, it said.
Tough sentencing laws, record numbers of drug offenders and high crime rates have contributed to the United States having the largest prison population and the highest rate of incarceration in the world, criminal justice experts say.
The new report said that the national prison population has nearly tripled between 1987 and 2007.
"The number of people behind bars in the United States continued to climb in 2007, saddling cash-strapped states with soaring costs they can ill afford and failing to have a clear impact either on recidivism or overall crime," it said.
States last year spent more than $44 billion (€30 billion) on corrections, the report said, compared with $10.6 billion (€7 billion) in 1987, the report said, adding that the rate of increase for prison costs was six times greater than for higher education spending.
The report said some states, such as Texas and Kansas, have acted to slow their prison population growth, with greater use of community supervision for lower-risk offenders and sanctions other than prison for minor offences.