Beginning in the 1970s, the United States began an experiment in mass imprisonment. Supporters argued that harsh punishments such as imprisonment reduce crime by deterring inmates from reoffending. Skeptics argued that imprisonment may have a criminogenic effect. The skeptics were right. Previous narrative reviews and meta-analyses concluded that the overall effect of imprisonment is null. Based on a much larger meta-analysis of 116 studies, the current analysis shows that custodial sanctions have no effect on reoffending or slightly increase it when compared with the effects of noncustodial sanctions such as probation. This finding is robust regardless of variations in methodological rigor, types of sanctions examined, and sociodemographic characteristics of samples. All sophisticated assessments of the research have independently reached the same conclusion. The null effect of custodial compared with noncustodial sanctions is considered a “criminological fact.” Incarceration cannot be justified on the grounds it affords public safety by decreasing recidivism. Prisons are unlikely to reduce reoffending unless they can be transformed into people-changing institutions on the basis of available evidence on what works organizationally to reform offenders.