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The Great California Prison Experiment

I published an academic paper back in 1996 that tried to measure the impact that changes in the prison population have on the crime rate. It turns out that this is a hard question.
You can’t just look across states and compare incarceration rates and crime rates, because the places with the worst crime problem will also tend to have the most prisoners.
Holding constant the punishments, the more crime that occurs, the greater the prison population. So correlations just are not very useful for determining whether prisons are effective or not.

“While those crime numbers sound bad, according to my estimates, letting out the prisoners is more or less a wash from a societal cost-benefit perspective.”

What you need is something more like a randomized experiment in which, for extraneous reasons, a whole bunch of prisoners get let out (or a whole bunch of extra criminals get locked up). For obvious reasons, the National Science Foundation would never give me a research grant to do that myself.
Indirectly, however, the A.C.L.U. (or in the California case, another prisoners’ rights group) does the next best thing. It brings lawsuits against state prison systems arguing that the prisons are so overcrowded that they represent “cruel and unusual” punishment. The A.C.L.U. virtually always wins these suits. The state appeals, and roughly a decade after the suit is filed, the court’s initial decision is upheld and the A.C.L.U. is victorious.
As I report in my paper, these lawsuits have a large impact on the prison populations in the affected states. After the suits are filed, but before any court decisions are handed down, prison populations grow more slowly in the litigation states.
The preliminary court decision doesn’t have much of an effect. But when the final verdict is handed down, prison populations shrink by about 15 percent relative to the rest of the country over the next three years.
Yesterday, a prisoners’ rights group won a preliminary decision against the state of California’s prison system. Consistent with my earlier results, the lawsuit already seems to have had some impact on California’s prison population. For instance, in 2007 California’s prison population shrank by about 1 percent, whereas the overall U.S. prison population grew by nearly 2 percent. It will take a few years before a final court decision is handed down, but the likely outcome is that five or six years from now there will be 25,000 fewer inmates than there otherwise would have been.
What does this mean for crime? If my estimates are correct, ultimately violent crime will be roughly 6 percent higher in California than it would have been absent the lawsuit. That is roughly 150 extra homicides a year, 500 additional rapes, and 4,500 more robberies.
While those crime numbers sound bad, according to my estimates, letting out the prisoners is more or less a wash from a societal cost-benefit perspective. The money we save from freeing the prisoners is on the same order of magnitude as the pain and suffering associated with the extra crime.
I do have one very specific policy recommendation to the state of California. If they do a mass release of prisoners, it should be done with strings attached. Namely, if the released prisoner gets convicted of a crime again in the future, his sentence the next time around should be whatever it normally would be plus all of the time that he should have served on his current sentence that gets cut short because of the early release.
This rule would strengthen the incentives for the ex-cons to stay straight. Italy enforced such a policy after a mass release, and it appears to have been quite effective.