But a general rule in economics — the law of diminishing marginal benefits — applies to incarcerating additional people or adding years to sentences. Research finds that more incarceration has, at best, only a small effect on crime because our incarceration rate is already so high. As the prison population gets larger, the additional prisoner is more likely to be a less risky, nonviolent offender, and the value of incarcerating him (or, less likely, her) is low.

The same general principle applies to the length of prison sentences, which in many cases have gotten longer as a result of sentence enhancements, repeat-offender laws, “three strikes” laws and “truth-in-sentencing” laws. Longer sentences do not appear to have a deterrent effect; one study finds, for example, that the threat of longer sentences has little impact on juvenile arrest rates. Other studies have found that sentencing enhancements have only modest effects on crime. They are unlikely to meaningfully affect the overall crime rate or generate meaningful gains in public safety.

Moreover, in many cases the analysis suggests that adding prisoners or years to sentences can be harmful. A growing body of research shows that incarceration and longer sentences could increase recidivism. Individuals may build criminal ties while incarcerated, lose their labor-market skills and confront substantial obstacles to re-entry after release. A new study finds that each additional year of incarceration increases the likelihood of re-offending by four to seven percentage points after release.

The bottom line: The putative benefits of more incarceration or longer sentences are actually costs.

Those costs are not confined to the prison population. Time in prison not only means a loss of freedom, but it also means a loss of earnings, risks to the health and safety of the incarcerated, and prolonged absences from family that can strain marriages and increase behavioral problems in children. The probability that a family is in poverty increases by nearly 40 percent while a father is incarcerated.