Prison sentences don't deter criminals from engaging in violent crime, a new study has found.
And keeping them behind bars has barely any effect on reducing violent crime overall.
Researchers in the US looked at data covering more than 110,000 convicted felons - some jailed, some sentenced to probation. They found while there was a slightly reduced chance of reoffending or arrest in the five years following their conviction, that was only because they were usually still in prison.
Once released, they reoffended at the same rates as those sentenced to probation.
"These results suggest that for individuals on the current policy margin between prison and probation, imprisonment is an ineffective long-term intervention for violence prevention, as it has - on balance - no rehabilitative or deterrent effects after release," the University of California Berkeley researchers concluded.
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The felonies were committed between 2003 and 2006 in Michigan, and the offenders' post-prison and post-probation behaviour followed through until 2015.
While offenders doing time are clearly limited in their ability to carry out further acts of violence, keeping them incarcerated still only had a negligible effect on crime rates. Looking at all violent crimes, the researchers found only 0.7 percent were prevented because the would-be offender was already behind bars.
"These calculations suggest that the estimated number of violent crimes prevented by imprisonment is small relative to the total amount of violent crime in the state," the study says.
"Our results suggest that imprisoning fewer people for violent offences on the margin between prison and probation would have relatively small effects on violence in the community and that alternative policies and interventions to prevent violence may show greater benefit at reduced social and economic costs."
The Michigan state penal system houses 40,000 inmates - four times as many prisoners as New Zealand, despite the state only having twice New Zealand's population. An additional 25,000 people are locked up in local jails and federal prisons.
New Zealand's prison population is falling, after a decade of going up - going from more than 10,500 last year to about 10,000 today.
The study was published in journal Nature Human Behaviour.