Locking away people who have committed assault, robbery and similar felonies may keep them off the streets for a period of time, but it does not affect whether they will commit violent crimes after their release, according to new research from UC Berkeley.

The findings, published online this week in the journal Nature Human Behaviour, challenge tough-on-crime measures, such as mandatory minimum sentences, and make a compelling case for the greater use of prison diversion programs for people who are eligible for probation.

“Our study shows negligible public safety gains are made from imprisoning individuals who are eligible for probation, and that those gains last only as long as the individual is in prison,” said study lead author David Harding, a UC Berkeley sociology professor.

Harding and fellow researchers sought to understand whether prison sentences lower the likelihood of offenders committing future violent crimes once freed.

They analyzed the post-release arrest and conviction records of more than 100,000 people in Michigan who had been convicted between 2003 and 2006 and followed their interactions with the criminal justice system through 2015.

Their analysis focused on cases in which judges had the discretion to either sentence defendants to prison or to probation. It excluded individuals who had committed such extreme violent crimes as rape and murder and who are ineligible for probation.

In comparing the ex-inmates’ post-release arrest and conviction records against the probation cohort, researchers found that those who had been imprisoned were just as likely as their peers on probation to be convicted of violent crimes within five years of their release.

“The takeaway here is that imprisonment doesn’t make much of a dent in violent crime rates,” Harding said.

Opponents of criminal justice reforms commonly argue that reduced or diverted prison sentences compromise public safety. Meanwhile, laws to reduce recidivism, such as the federal First Step Act, enacted in December 2018, apply primarily to those convicted of non-violent crimes.

At least 1.5 million people are incarcerated in American federal and state correctional facilities at an annual cost of tens of billions of dollars to taxpayers. About half of prisoners have been convicted of a violent offense.

“We are spending a lot of money on imprisonment for very little benefit in terms of public safety,” said Harding. “Our findings show we could incarcerate fewer people convicted of violent crimes and invest the savings in other ways of preventing violence in society.”

In addition to Harding, co-authors of the study are Jeffrey Morenoff at the University of Michigan, Anh Nguyen at the Kaiser Permanente Colorado Institute for Health Research, Shawn Bushway at the State University of New York at Albany and Ingrid Binswanger at Kaiser Permanente Colorado Institute for Health Research and the University of Colorado.