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.