Violence is a serious public health problem. There are significant healthcare costs associated with injuries caused by force. Individuals involved in violence can suffer lasting physical and mental health effects. The impacts of violence can also extend to their families, friends, caregivers, and communities.

What can be done to prevent violence? The answer is often prison. In fact, in the United States, half of the people in prisons are there because of a violent offense. It's thought that sending people convicted of violent crimes to prison will prevent future violence in the community. But it's unclear to what extent this appears to be true.

Prison can reduce future violence in the community in several ways. The experience of imprisonment may be so unpleasant that it will deter individuals from committing violent acts after release to avoid going back. People in prison may receive treatment for underlying mental illnesses that may have contributed to past violent behavior. They also cannot commit acts of violence against people in the community while they are in prison.

It's also possible that prison might increase future violence in the community. The stresses of life in prison can worsen pre-existing mental illnesses that contribute to violence. Or may even trigger new ones. Individuals may learn to rely on aggression to defend against being victimized while in prison. It can be challenging to leave behind this strategy after release. Prisons are often in isolated areas, far away from family, friends, and other sources of support. It may be hard then to maintain these connections, which are crucial for successfully reintegrating into society and avoiding crime after release. People leaving prison may also face barriers to finding housing, employment, and healthcare because of their time in prison and criminal record. These challenges can push them further to the margins of society and encourage the formation of pro-criminal social networks.

In our recently published study, we investigated how prison affects violent crime by comparing people sentenced to imprisonment to those sentenced to probation supervision in the community. The study focused on people who were convicted of a violent crime and were eligible for both prison or probation. These people were typically convicted of assault and robbery. Those who were convicted of more serious violent crimes like rape and murder are generally not eligible for probation.

It's challenging to assess whether prison reduces or increases future violence. Those who are likely to engage in violent acts are also more likely to be sentenced to prison. It's difficult to tell whether future violent acts are a result of going to prison itself or something related to those who end up in prison.

To overcome this problem, we use a novel research design that acts as an experiment. We leverage the fact that judges are randomly assigned to criminal cases. Some judges prefer to send people to prison, while others prefer to put them on probation. So, depending on the luck of the draw, individuals may be sentenced to prison or probation.

Our results show that sentencing someone to prison instead of probation did not affect their likelihood of being convicted of a violent crime within five years after release. Even in the short-term, prison prevents little violent crime during the time that individuals are held behind bars. For every person prevented from committing a new violent felony within five years of their sentence, we would need to imprison another 16.

If one of the goals of prison is to prevent future violence in the community, our study suggests that it's an ineffective long-term solution. It neither rehabilitates individuals nor deters them from committing future violent acts.

In addition to little violence prevention, there have been growing concerns about the public health and social costs of imprisonment. People who leave prison face a heightened risk of death. Often because they become victims of violence, those returning from prison must also grapple with the challenges of finding a stable home and job. Both are important for avoiding crime but are often out of reach for people leaving prison. The negative consequences of imprisonment can also impact children, partners, and family members.

The economic costs of keeping a person in prison are also enormous. Because keeping people in prison prevents minimal violent crimes, we should send fewer individuals convicted of violent offenses to imprisonment. They could otherwise serve their time in the community and the savings could be invested in more effective interventions to prevent violence.

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