WASHINGTON — The number of prisoners serving life sentences has increased 83 percent in the past 10 years as tough-on-crime initiatives have led to harsher penalties, a study says.

Nearly 128,000 people, or one of every 11 offenders in state and federal prisons, are serving life sentences, according to the study released Tuesday by The Sentencing Project, a Washington-based group that promotes alternatives to prison. In 1992, 70,000 people had life sentences.

The figures, compiled from the Federal Bureau of Prisons and state correctional agencies, also show the amount of time served by criminals given life sentences increased from an average of 21 years to 29 years between 1991 and 1997.

The report said the increases are not the result of more crime, since violent crime fell significantly during the period covered by the study. Rather, longer mandatory sentences and more restrictive parole and commutation policies are most responsible.

In Tennessee, for example, state law requires that any person sentenced to life with the possibility of parole serve at least 51 years before release is considered.

In Pennsylvania, all life sentences have been imposed without parole since the 1940s, but governors frequently commuted such sentences, doing so in more than 300 cases in the 1970s. But only one lifer has had a sentence commuted since 1995, the report said.

The report cites one-size-fits-all "three strikes" laws requiring life sentences for any third felony conviction as key to boosting the number of lifers. Many of those given such penalties are nonviolent drug offenders.

"The people serving life have committed serious offenses, but it doesn't mean that imposing life sentences across the board is always appropriate or the best crime control strategy," said Mark Mauer, assistant director of The Sentencing Project and co-author of the study.

New York had the highest percentage of its state inmates serving life sentences, 19.4 percent, followed by Nevada, 18.6 percent; California, 18.1 percent; Alabama, 17.3 percent; and Massachusetts, 16.9 percent.

Kent Scheidegger, legal director of the Criminal Justice Legal Foundation in California, which favors stiff sentences, said people who commit serious crimes should not be treated easily.

"There's a good reason for tough sentencing," he said. "It's to keep the bad guys off the street so they can't commit crimes.

"For the worst of murders the appropriate sentences are life without parole and death," he said. "If they've gotten life without parole, they've gotten off easy."

In 2003, one in four lifers was serving without the possibility of parole; in 1992 it was one in every six, according to the report. The study also found that as of 1997, 90 percent of those serving life sentences were in prison for a violent offense, including 69 percent for murder.

"We can't say across the board none of them should have life sentences, and conversely that the 90 percent that are in for violent crimes should be in for life," Mauer said. "They should be assessed individually in terms of sentencing and then considered if they should be released by a parole board or some other decision-making body."

The report details how tougher standards have swollen the population of lifers, further straining the resources and capacity of state prison systems.

It costs $1 million to house a person sentenced to life in prison for 40 years, according to the report. Mauer said that money could in some cases be better spent on preschool education, rehabilitation or more police officers.

Michael O'Neill, law professor at George Mason University and a member of the U.S. Sentencing Commission, which sets guidelines for federal judges, said a life sentence is an important weapon against crime.

"If you take away life sentences, you reduce one of the important deterrent effects," he said.

Still, O'Neill said, prison sentences at the state and federal levels should be reviewed to make sure the penalty fits the crime.

"Incarceration of habitually violent offenders is a good thing because it prevents them from preying on society," O'Neill said. "But it is less clear whether long-term prison sentences are warranted for drug offenders — not kingpins, but low-level drug offenders. The jury is out."