One of us is a former Treasury secretary, the other directs a criminal-justice institute. But we’ve reached the same conclusions. America’s overreliance on incarceration is exacting excessive costs on individuals and communities, as well as on the national economy. Sentences are too long, and parole and probation policies too inflexible. There is too little rehabilitation in prison and inadequate support for life after prison.

Crime itself has a terrible human cost and a serious economic cost. But appropriate punishment for...