State prison populations declined slightly last year after growing for nearly four decades, says a new study from the Pew Center on the States reported by the New York Times. The likely reasons for the shift are efforts to keep parolees out of prison and cutting time served by nonviolent offenders. “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 Pew’s Pubic Safety Performance Project.Marc Mauer of the Sentencing Project, an advocacy group, called the reduction overdue, given that 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. Criminologists say that the high incarceration rate is one reason that crime has dropped, but that the imprisonment level is responsible for no more than 25 percent of the decrease. [An earlier version of this story, based on an incorrect New York Times account, gave the wrong percentage for the state prison population decline.]