State prison populations, which have grown for nearly four decades, have begun to dip, according to a new report, largely because of recent efforts to keep parolees out of prison and reduce prison time for nonviolent offenders.

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A Slight Drop in Inmate PopulationState prisons held 1,403,091 people as of Jan. 1, nearly four-tenths of a percent fewer than a year before, the report said. Prison populations have fallen in 27 states in that period, while they have risen in 23.

“It’s too early to tell whether this is a tap of the brakes or a shift into reverse,” said Adam Gelb, the director of the public safety performance project of the Pew Center on the States in Washington, which produced the report. Still, Mr. Gelb said, seeing the state prison numbers dip for the first time since 1972 “took us a little bit by surprise,” he said.

In the same period, the population in federal prisons increased by nearly 3.4 percent.

The results broaden the conclusions in a report issued this month by the Sentencing Project, a research and advocacy group in Washington that looked at efforts to reduce the prison populations in Kansas, Michigan, New Jersey and New York. That report found that all four states had achieved reductions, with New York reaching a 20 percent reduction and New Jersey 19 percent over a decade.

Marc Mauer, the executive director of that group, said the reduction was actually overdue, since crime rates have declined for some 15 years. “That’s the puzzling piece — why did this take so long?” he asked. The lag, he said, was partly the result of longer sentences and partly because of tough standards in many states for revoking parole.

The Pew report noted that while the squeeze on state and local budgets had contributed to efforts to reduce prison populations, “financial pressures alone do not explain the decline.” At least part of the fall-off resulted from changes like California’s decision to reduce the number of low-risk people on parole returning to prison because of technical violations, and Texas’ decision to step up its residential and community-based treatment programs.

“If you had to single out the most common reform that we’re seeing,” Mr. Gelb said, “it’s various strategies to hold parole violators accountable, short of jamming them back into a $25,000-a-year, taxpayer-funded prison cell.”

Releases of prisoners, however, have been controversial. Crime Victims United of California, a nonprofit group, sued the state last month over its efforts to reduce the number of inmates in its prisons, claiming that releases driven by overcrowding would violate a 2008 voter initiative.

The new report does not deal with the prisoner levels in local jails. A 2009 report by the Pew center that did count local jail inmates concluded that 1 in 100 adults in the United States lives behind bars.

The new report concluded that whatever the long-term trends, with 1.6 million people in state and federal prisons and an estimated 700,000 in local jails, “the United States will continue to lead the world in incarceration for the foreseeable future.”