California's prison population declined in 2009 for the third straight year as the number of state prisoners fell nationally for the first time in nearly four decades, according to a new survey from the Pew Center on the States.

The overall decline was relatively small, 0.4% of roughly 1.4 million state inmates in the nation, but the study's authors said it is significant because it represents the first year-over-year drop since 1972.

"After so many years on the rise, any size drop is notable," said Adam Gelb, director of the Pew Center's Public Safety Performance Project. "Most people thought that the prison count was just going to continue up and up and up."

California reported the greatest absolute drop last year -- a reduction of 4,257 prisoners. The continuing decline represents a trend for California, where the number of inmates grew from 76,000 in 1988 to nearly 170,000 today.

The number of incarcerated Californians had surged in the two previous decades, as voters and lawmakers approved tough-on-crime measures such as the "three-strikes" law. The costs of state prisons has soared as well, doubling since 2000.

What changed?

"There is not a clear explanation for why California dropped as much as it did," Gelb said.

One big downward pressure on California's inmate population is a federal court order to cut the prison population by as many as 40,000 inmates. Another has been the state's outsized deficits, which has led lawmakers to try to trim money from the corrections system.

Last year, legislators reduced parole supervision for low-level offenders and approved taking up to six weeks off prison terms for inmates who complete rehabilitation programs.

And state corrections and parole officials have tried to slow the revolving door of prisoners who leave lockups only to return months later after violating parole.

"I wouldn't say it's any one thing," said Gordon Hinkle, spokesman for the California Department of Corrections and Rehabilitation.

Other states reported mixed results.

The number of inmates declined in 27 states and grew in 23, according to the Pew study.

Rhode Island reported the greatest percentage population drop, 9.2%; Indiana reported the largest percentage increase, 5.3%.