Previous posts in this series have examined the latest available incarceration data from Indiana, Minnesota, and Wisconsin. This post considers historical data. I’m particularly interested in the impact of a major change in sentencing law that was adopted in Wisconsin in 1998. Under the “truth in sentencing” law, parole was abolished for crimes committed on or after December 31, 1999. What impact did this have on the size of the state’s prison population? Two hypotheses occur to me. First, if judges continued to impose the same nominal sentences that they had been imposing, one would expect the prison population to grow because offenders would be serving longer real sentences. Alternatively, judges might have reduced their nominal sentences to account for the loss of parole release options, attempting thereby to achieve the same real sentences as before TIS; such discounting would presumably lead to stability in the imprisonment rate.

The data, set forth in the table below, seem to support the latter hypothesis, with the current rate of imprisonment almost exactly matching that of 2000, the first full year after TIS took effect. Indeed, since 1999, the state’s imprisonment rate has been remarkably stable. The single largest annual change since 1999 was a 5.8% drop in 2005. This makes for quite a contrast with the volatile 1992-1999 time period, when annual increases averaged 12%.

The picture becomes even more interesting if we focus on Wisconsin’s imprisonment rate relative to that of peer states Indiana and Minnesota.

Since TIS, Wisconsin’s imprisonment rate has dropped markedly in comparison with those of the peer states. In 1999, Wisconsin had an imprisonment rate 21% higher than Indiana’s, while the current rate is only 84% of the Hoosier State’s. Likewise, in 1999, Wisconsin’s imprisonment rate was more than triple Minnesota’s, but is now only a little more than twice that of its neighbor to the west.

Wisconsin’s strong improvement in imprisonment rate relative to its peer states does not seem merely a function of more significant gains in reducing violent crime. Although Wisconsin has reduced its violent crime rate most years (eight out of eleven) since 1999, Indiana has been no less consistently successful on this front, and Minnesota only a little less so (six years out of eleven). Indeed, Wisconsin’s rate of violent crime in 2010 was actually slightly higher than it was in 1999 (248.7 versus 245.8), while Minnesota’s (236.0 versus 274.0) and Indiana’s (314.5 versus 374.6) were much lower.

If not crime rate, what else might explain Wisconsin’s greater success in holding the line on imprisonment for the past decade than Indiana or Minnesota? Has TIS itself played a role? It is hard not to think so in light of the abrupt break that occurred in 2000 in what had been an established pattern of large annual increases in Wisconsin’s imprisonment rate.

It is not immediately clear why TIS would have put the brakes on a rapidly expanding prison population. One possibility is that sentencing judges in the tough-on-crime 1990’s were overestimating, and overcompensating for, the lenience of the parole board. Perhaps the parole board itself was something of a moving target, tightening up its standards in the ‘90’s in ways that were not understood by sentencing judges. If so, then the dramatic growth of the state’s prison population may have been in part due to a communication and coordination problem between the judiciary and the parole board. By taking the parole board out of the equation for new crimes, TIS may have mitigated a dysfunctional institutional dynamic.

Cross posted at Life Sentences Blog.