There are five times as many people in prison today—nearly 5% of the population will be imprisoned at some point—as there were in the 1970s. The increase in crime during the 1960s and ’70s motivated Americans to get tough on crime, which took several forms. The most striking of these was putting lots of people in prison. Imprisonment is supposed to reduce crime in two ways: it takes criminals off the street so they can’t commit new crimes (incapacitation) and it discourages would-be criminals from committing crime (deterrence).

But neither of these outcomes came to pass.

A new paper from University of Michigan economics professor Michael Mueller-Smith measures how much incapacitation reduced crime. He looked at court records from Harris County, Texas from 1980 to 2009. Mueller-Smith observed that in Harris County people charged with similar crimes received totally different sentences depending on the judge to whom they were randomly assigned. Mueller-Smith then tracked what happened to these prisoners. He estimated that each year in prison increases the odds that a prisoner would reoffend by 5.6% a quarter. Even people who went to prison for lesser crimes wound up committing more serious offenses subsequently, the more time they spent in prison. His conclusion: Any benefit from taking criminals out of the general population is more than off-set by the increase in crime from turning small offenders into career criminals.

More here, based on this paper (PDF) by Michael Mueller-Smith.

Why does prison turn a one-time criminal into a lifelong one? Those coming out of prison have few other attractive options.

Prison obliterates your earnings potential. Being a convicted felon disqualifies you from certain jobs, housing, or voting. Mueller-Smith estimates that each year in prison reduces the odds of post-release employment by 24% and increases the odds you’ll live on public assistance. Time in prison also lowers the odds you’ll get or stay married. Being in prison and out of the labor force degrades legitimate skills and exposes you to criminal skills and a criminal network. This makes crime a more attractive alternative upon release, even if you run a high risk of returning to prison.