Of all of the notions that have motivated the decades-long rise of incarceration in the United States, this is probably the most basic: When we put people behind bars, they can't commit crime.
The implied corollary: If we let them out, they will.
By this thinking, our streets are safer the more people we lock up and the longer we keep them there. This logic suggests that there would be serious public-safety costs to reducing prison populations, a policy in the news again after the U.S. Sentencing Commission unanimously voted last Friday to retroactively extend new, lighter drug sentencing guidelines to about 46,000 offenders currently serving for federal drug crimes. As the National Association of Assistant U.S. Attorneys warned, opposing the move, "tough sentencing laws . . . led to safer communities, which are now threatened."
Crime trends in a few states that have significantly reduced their prison populations, though, contradict this fear. Here's a picture of the recent decline in state prison populations in New York and New Jersey, from a new report by the Sentencing Project:
And here are crime trends in those two states over the same time:
It's important to note that crime has been falling all over the country over this same time, for reasons that are not entirely understood (and, no, not entirely explained by the rise of incarceration). But the Sentencing Project points out that declining violent crime rates in New York and New Jersey have actually outpaced the national trend, even as these states have reduced their prison populations through changing law enforcement and sentencing policies.
We certainly can't take these three charts and conclude that reducing prison populations reduces crime. But these trends do make it harder to argue the opposite — particularly in the most heavily incarcerated country in the world. As the Sentencing Project puts it, "in the era of mass incarceration, there is a growing consensus that current levels of incarceration place the nation well past the point of diminishing returns in crime control."