The report said growth in prison numbers had not been driven by a similar increase in crime rates or a corresponding increase in the nation's population.
 
"Rather, it flows principally from a wave of policy choices that are sending more lawbreakers to prison and, through the popular 'three-strikes' measures and other sentencing enhancements, keeping them there longer," it said.
 
Correction expenditure
 
US states spent more than $44bn on corrections last year, the report said, compared with $10.6bn in 1987.
 
Ryan King of the Sentencing Project, a US prison reform group, told Al Jazeera that many of those currently incarcerated were serving sentences for minor offences or were drug users.
 
"We are using tens of billions of dollars of our domestic resources to incarcerate individuals who would be much better off either under community supervision or in a public health treatment programme."
 
The report said that the national prison population had almost tripled between 1987 and 2007.
 
While one in 106 adult white men are incarcerated, one in 36 Hispanics and one in 15 African-Americans are behind bars.
 
Young black men
 
Younger black men fare even worse, with one in nine African-Americans ages 20 to 34 held behind bars.
 
King said this was the case because US law-enforcement agencies chose to enforce drug laws more strictly among African-Americans.
 
"African-Americans are more likely to be arrested because law enforcement is centred in those communities."
 
While men are still 10 times more likely to be incarcerated than women, the female penal population is "burgeoning at a far brisker pace", the study said.
 
Some states, such as Texas and Kansas, had slowed their prison population growth, with more use of community supervision for lower-risk offenders and use of sentences other prison.