The US prison population almost trebled between 1987 and 2007

The Pew Center report calls the US the global leader in the rate at which it imprisons its citizens.

Over 2.3 million people were being held this year, it said - far ahead of other countries with large prison populations like China, Russia and Iran.

The report called for fewer low-risk offenders to be sent to jail.

Soaring costs

It claims that the growing prison population "is saddling cash-strapped states with soaring costs they can ill afford, and failing to have a clear impact either on recidivism or overall crime".

With 750 inmates per 100,000 people, imprisonment cost the 50 states more than $49bn last year, up from less than $11bn 20 years earlier.

The rate of increase for prison costs was found to be six times greater than for higher education spending.

RATES OF INCARCERATION USA: 750 prisoners per 100,000 people South Africa: 341 per 100,000 Iran: 222 per 100,000 China: 119 per 100,000

According to the Pew Center study, the higher rates of incarceration did not reflect a similar increase in crime, or in population, but tougher sentencing measures.

Some states, though, 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 probation and parole violations.

Disproportionate numbers

The numbers were "especially startling", according to the Pew Center report, for some groups in the population.

"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 total of 2.3 million adults held in prison - or one in every 99.1 adults - puts the US far head of other countries.

China, with its far greater population, has 1.5 million people behind bars, and Russia has 890,000.