A team of researchers from the University of California, the University of Michigan, Kaiser Permanente Colorado Institute for Health Research, the State University of New York and the University of Colorado School of Medicine has found evidence that incarcerating people who commit serious crimes does not prevent them from committing more crimes once they are released. In their paper published in the journal Nature Human Behavior, the researchers describe a study they conducted using statistics from people incarcerated in Michigan prisons for committing violent crimes, and what they found.

Locking people up when they have been convicted of a crime is an age-old form of punishment. In the short term, it prevents the offender from committing more crime—at least against those outside the prison gates. But jailing people has also been espoused as a means of teaching the offender a lesson—being locked up is supposed to make them think twice about committing future crimes once they are released. But does it? That is what the researchers sought to find out.

To learn more about the probability of engaging in criminal activity after release from prison, the researchers looked at data for 110,000 people convicted of violence-related felonies during the years 2003 to 2006 in Michigan—some had been sent to prison, others we given probation. The researchers followed the records through the year 2015 looking for examples of arrests or incarcerations.

The researchers report that they saw a slight decrease in crime for those sent to prison compared to those who received probation, but only for the time they were in prison. After they were released, they were found to be just as likely to engage in crime as those who had been given probationary sentences. The data indicates that serving time in prison did not serve as a deterrent for those convicted of a crime. The researchers suggest imprisonment is an ineffective deterrent, and because of that, policymakers ought to take at a closer look at its use. Putting people in prison, they note, is a lot more expensive than probation.