One of the major justifications for the rise of mass incarceration in the United States is that placing offenders behind bars reduces recidivism by teaching them that “crime does not pay.” This rationale is based on the view that custodial sanctions are uniquely painful and thus exact a higher cost than noncustodial sanctions. An alternative position, developed mainly by criminologists, is that imprisonment is not simply a “cost” but also a social experience that deepens illegal involvement. Using an evidence-based approach, we conclude that there is little evidence that prisons reduce recidivism and at least some evidence to suggest that they have a criminogenic effect. The policy implications of this finding are significant, for it means that beyond crime saved through incapacitation, the use of custodial sanctions may have the unanticipated consequence of making society less safe.