“Wisconsin prison population 2.5 times larger than Minnesota’s”

That’s the headline of this local newspaper report on a study comparing the prison populations of the neighboring Midwestern states. The study offers an interesting natural experiment on how much policy decisions — as opposed to sociological factors or crime rate — drive mass incarceration. Minnesota and Wisconsin have roughly similar demographics, geography, size (~ 5 million people), crime rates, etc. (though I’m sure native Midwesterners could point out differences I’m overlooking, they seem similar enough to compare), and yet the two have taken vastly divergent paths:

In 2008, Wisconsin had more than 23,000 people in state prisons, compared to fewer than 9,000 in Minnesota. Yet, despite the marked difference in prison populations, Minnesota had more residents under some form of correctional control–prison, jail, probation, or parole. …

Wisconsin’s larger prison population primarily reflects events that occurred from 1993 to 1999. During that period, Wisconsin added more than six times as many prisoners as Minnesota did. One factor in the rising number of prisoners here was parole changes. Due to heightened visibility of crime as a policy issue, public concern over insufficient prison terms, and resulting political pressure, Wisconsin’s willingness to parole prisoners diminished. According to some estimates, the change effectively extended prison sentences by 16% to 18%.

Taking a different approach, Minnesota passed a community corrections law in the early 1970s and felony sentencing guidelines in the 1980s in an effort to contain the state’s prison population. As a result, growth in Wisconsin’s prison population was two times as fast as Minnesota’s in the 1990s. However, from 1999 through 2008, the number of prisoners in Minnesota rose 50.8%, compared with 9.7% in Wisconsin.

You can also see the difference by comparing Minnesota and Wisconsin using the Sentencing Project’s interactive map, which shows that Minnesota incarcerates 137 per 100,000 residents, while Wisconsin incarcerates 374 per 100,000. Wisconsin’s rate is still well below high-incarceration Southern states (which incarcerate 600-800 per 100,000), and it’s not particularly high for the Midwest, but the comparison with Minnesota suggests it could easily be lower with some targeted policy reforms.