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The prison population in the United States dropped in 2012 for the third consecutive year, according to federal statistics released on Thursday, in what criminal justice experts said was the biggest decline in the nation’s recent history, signaling a shift away from an almost four-decade policy of mass imprisonment.

The number of inmates in state and federal prisons decreased by 1.7 percent, to an estimated 1,571,013 in 2012 from 1,598,783 in 2011, according to figures released by the Bureau of Justice Statistics, an arm of the Justice Department. Although the percentage decline appeared small, the fact that it followed decreases in 2011 and 2010 offers persuasive evidence of what some experts say is a “sea change” in America’s approach to criminal punishment.

“This is the beginning of the end of mass incarceration,” said Natasha Frost, associate dean of Northeastern University’s school of criminology and criminal justice.

About half the 2012 decline — 15,035 prisoners — occurred in California, which has decreased its prison population in response to a Supreme Court order to relieve prison overcrowding. But eight other states, including New York, Florida, Virginia and North Carolina, showed substantial decreases, of more than 1,000 inmates, and more than half the states reported some drop in the number of prisoners. (Figures for three states were estimated because they had not submitted data in time for the report.) The population of federal prisons increased slightly, but at a slower rate than in previous years, the report found.