Whether punishment promotes or deters future criminal activity by the convicted offender is a key public policy concern. Longer prison sentences further isolate offenders from the legitimate labor force and may promote the formation of criminal networks in prison. On the other hand, greater initial punishment may have a deterrence effect on the individual being punished, sometimes called "specific deterrence," through learning or the rehabilitative effect of prison.
We test the effect of prison sentence length on recidivism by exploiting a unique quasi-experimental design from adult sentences within a courthouse in Seattle, Washington. Offenders who plead guilty are randomly assigned to a sentencing judge, which leads to random differences in prison sentence length depending on the sentencing judge's proclivities. We find that one-month extra prison sentence reduces the rate of recidivism by about one percentage point, with possibly larger effects for those with limited criminal histories. However, the reduction in recidivism comes almost entirely in the first year of release, which we interpret as consistent with prison's rehabilitative role.
That's one item, but the argument that prison reduces crime is far more robust than that.