The number of prisoners serving life terms in the United States has more than quadrupled since 1984, and so-called lifers now account for one in nine people behind bars, according to a report released on Wednesday by the Sentencing Project.
The Washington-based research and advocacy group, which has long pushed for criminal justice reforms, said in the report that nearly 160,000 people were serving life sentences in 2012, or 10.6 percent of the 1.5 million inmates being held in state and federal prisons.
"Life sentences have increased steadily over the years beginning with the first documented national census of this population in 1984," the report said, adding that the total was up nearly 12 percent since 2008.
Nonwhites make up nearly two-thirds of the total population serving life sentences, with African Americans accounting for nearly half, the report said.
The Sentencing Project and other groups, including the London-based International Centre for Prison Studies, say the United States leads the world in the percentage of its population behind bars.
In addition to the overall increase in the number of inmates serving life sentences over the years, the report said there had been a 300 percent rise in the number of prisoners serving life without parole since 1984.
That same population - of lifers condemned to sentences without parole - swelled to nearly 50,000 prisoners last year, and the number has increased more than 22 percent just since 2008.
The reasons cited in the report include what it called the growing popularity of life sentences in the United States since a ban on the death penalty that was in place from 1972 to 1976.
Mandatory minimum sentencing and related laws enacted in the 1980s and 1990s, a time of rising crime and drug violence, are also factors, it said.
According to the Project, Florida led the nation with a disproportionate total of nearly 8,000 prisoners serving life without parole last year. Together with Pennsylvania, Louisiana, California and Michigan, the state helped account for about 58 percent of all life without parole sentences nationally.
The report did not single out any one factor contributing to the outsized role played by the five states.
California earned a reputation for being especially tough on crime under former Governor Gray Davis in 1999-2003. Davis famously announced that on his watch individuals convicted of homicide would only leave prison "in a pine box."
The Sentencing Project said the most recent increases in life sentences have come against the backdrop of a substantial, long-term drop in serious crime, and modest declines in overall prison populations across the country.
"The upward creep in life sentences has accelerated in recent decades an element of the 'tough on crime' political environment that began in the 1980s," the report said.
"Today, as diverse coalitions of lawmakers and stakeholders are engaged in meaningful discussions about the role of corrections, lifers continue to be largely excluded from the discussion of sentencing reform."
Ashley Nellis, the report's author and a senior research analyst at the Sentencing Project, said she and other advocates for reform of the criminal and juvenile justice system were encouraged by steps the Obama administration announced last month to reduce sentences for nonviolent drug offenders.
Attorney General Eric Holder, the top U.S. law enforcement official, unveiled the proposals on Aug. 12, saying they were designed to bypass tough mandatory prison terms in order to reduce America's huge prison population.
Holder said the United States accounts for just 5 percent of the world's population but incarcerates almost a quarter of the prisoners around the globe.
The administration's proposals are unlikely to affect many people already in prison, however, and Nellis said she and others calling for reforms of the justice system see life without parole as "the worse of two evils" compared with the death penalty.
"We oppose life without parole across the board," Nellis told Reuters.
"That doesn't mean that we think that everyone should eventually get out of prison. We just think that everybody should eventually get a second look." If that were to happen, she said, "we will see that some people clearly need to stay in prison and other people are ready to go or have earned their release."
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