Pennsylvania had the largest prison population growth in the Northeast last year, part of a national trend of proliferating prison populations in which more than one in 100 American adults now is incarcerated, according to a study released yesterday by the Pew Center's Public Safety Performance Project.

In spite of the sobering statistics behind the highest imprisonment rate in American history, the report concluded that efforts to reduce incarceration and recidivism have proved effective in some states.

"We've crossed this statistical and psychological threshold, but we don't have to keep heading down this path," said the project's director, Adam Gelb, in a news teleconference.

Pennsylvania added about 1,600 prisoners to its state prison population in 2007 -- a 3.7 percent increase from the previous year -- and totaled 46,028 on Jan. 1.

While that was the highest numerical growth in the Northeast, it was far behind increases in Florida's and Georgia's incarcerations, which grew by 4,447 and 2,413, respectively.

Texas' prison population -- 171,790 -- is the nation's highest, according to the study.

The study concluded that much of the growth in prison populations has to do with "a wave of policy choices that are sending more lawbreakers to prison and, through popular 'three-strikes' measures and other sentencing enhancements, keeping them there longer."

Alfred Blumstein, a criminologist at Carnegie Mellon University, said it has a lot to do with politics.

In the early '80s, Dr. Blumstein predicted that the prison population in Pennsylvania would continue to rise until the end of the decade and flatline at about 8,000 inmates.

Today, there are about 46,000 inmates. What he did not anticipate, he said, was a dramatic shift in the political paradigm in the early '90s that led politicians to push for stricter penalties for all offenders, bloating the prison population.

"The political system took over," he said. "As the public became increasingly concerned about crime, the politicians and particularly the Legislature responded by instituting a variety of tougher legislation such as mandatory minimum sentencing.

"It doesn't necessarily work in reducing crime but it did work in satiating the public," he said.

The study included some startling statistics.

Men are about 10 times more likely to be incarcerated, but the female population is growing at a faster rate. The study also found that age limits jail time. One in every 53 people in their 20s is in prison, but above age 55 that falls to one in 837. Even so, between 1992 and 2001, the number of state and federal inmates aged 50 and older rose from 41,586 to 113,358, a jump of 173 percent.

The racial disparity is stark: While one in 30 men between the ages of 20 and 34 is behind bars, for black males in that same age group the figure is one in nine.

Susan McNaughton, a spokeswoman for the state secretary of corrections, said parole violators and low-level offenders are behind the growth in Pennsylvania's prison population. She said that substance abuse is at the root of many of the inmates' "criminal thinking." Since the prison system traditionally might not treat their substance abuse problems, those inmates are likely to re-offend, she said.

Experts warn that the trend of a rising prison population is taking a heavy toll on states' budgets. In 1987, states collectively spent $10.6 billion of their general funds on corrections. Last year, the figure was $44 billion.

Last year, Pennsylvania spent about $1.6 billion on corrections or 6.2 percent of the general fund, according to the study. And yesterday, the state secretary of corrections requested an additional $600 million to build two or three new prisons to accommodate new prisoners in a system that's over capacity.

The study reports that for every dollar spent on higher education, Pennsylvania spent about 81 cents on corrections. Only five states spent more. By comparison, West Virginia spent 36 cents on corrections for each dollar on higher education.

"The point of the comparison ... is to point out that correction spending will ultimately begin to crowd out some of the optional spending," said Sue Urahn, managing director of the Pew Center on the States.

The study recommended diversion sentencing, including treatment programs and halfway houses, for low-risk offenders and increasing incentives for inmates to shorten their sentences through education and treatment programs. Such programs have successfully reduced prison populations in Texas, Kansas and Nevada.

The state Department of Corrections is recommending legislation to allow the department to re-examine certain criminals and recommend them for state intermediate punishment, reducing their time in prison. To be eligible, an offender would have to be recommended by both the judge and the prosecutor.

"What really works for these kinds of offenders is to get them into treatment rather than sending them to prison for a long period of time," said Ms. McNaughton. "That will save up our real expensive cell space."

Such programs can be a hard sell, Dr. Blumstein said, because politicians are afraid of appearing "soft on crime."

But the culture is slowly changing, nationwide and in Pennsylvania.