Does putting more people in prison markedly reduce crime outside prison walls—at least while those people are still in prison? I think that in writing my full report, I approached the research on this question with just as much skepticism as I did with deterrence. Yet the incapacitation research better withstood my scrutiny. I am convinced that decarceration on the scale proponents hope for measurably increases crime in the short run.

In 2001, the Maryland State Commission on Criminal Sentencing Policy, which gives judges nonbinding guidance on sentencing, modified a recommendation in a way that caused people of a certain age to be sentenced to significantly less time in prison. In particular, the commission lowered the age at which, when counting prior offenses, judges should stop considering a defendant’s juvenile record. Since judges generally sentence people with more priors to more time, and since the juvenile record age limit fell from 26 to 23 years, 23-25-year-olds with juvenile priors saw those erased from consideration, and received shorter sentences. In a study exploiting this sudden change, Emily Owens calculates that time served for this group fell an average 222 days.

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