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The theory: putting more people in prison helped reduce crime

The case for: It seems intuitive. The incarceration rate’s been rising; the crime rate’s been falling. Surely this is because people are being locked up who’d otherwise be committing crimes out on the streets.

Several academic studies have found that increased incarceration had a big impact on reducing crime. In particular, Steven Levitt (of Freakonomics fame) wrote a paper in 2004 that concluded that 58 percent of the drop in violent crime during the 1990s was due to incarceration.

The case against: These studies were based on older data that only included a few years of the crime decline. Levitt acknowledged he couldn’t account for the point of diminishing returns: There are only so many serious criminals out there, and after a certain point the people getting put in prison aren’t people who’d be committing crime after crime on the street. The higher the incarceration rate gets, the less it matters if you increase that rate even more. Studies that examine more recent data, after the point of diminishing returns has been hit, find that incarceration wasn’t nearly as influential.

”Incarcerating violent people has a big effect on violence,” John Roman, senior fellow at the Urban Institute’s Justice Policy Center, said. “But most people we incarcerate aren’t violent.”

incarceration diminishing returns Brennan Center for Justice

The diminishing returns aren’t only about who’s being put in prison, but about how long people remain there. The research suggests that people age out of crime, so letting them out of prison 10 or 20 years down the line — instead of the longer sentences applied today — might not pose a threat to public safety. “Crime is a young man’s endeavor,” Brian Elderbroom, senior fellow at the Urban Institute’s Justice Policy Center, said in December. “It’s not surprising that someone who commits a crime at a young age would be a completely different person by the time they’re in their 30s.”

The other problem with this theory is that incarceration rates were increasing for years before crime started going down.

incarceration crime Brennan Center for Justice

The bottom line: Some effect. Criminologists now tend to believe that incarceration accounts for a fraction of the drop in crime (say, 25 percent), but no more. A 2015 Brennan Center for Justice report estimates that incarceration played even less of a role than that, especially when it came to violent crime. The Brennan Center concluded that the rising incarceration rates through the 1980s had already locked up the truly violent criminals, and the point of diminishing returns was hit even before the crime rate started to fall.