Why are state prison populations shrinking? A research note

After four decades of explosive growth, the US imprisonment rate began to inch downward in 2008. Despite fostering extensive public and policy debate, we know surprisingly little about why state imprisonment rates are declining. Although prior studies have examined correlates of decreases in imprisonment, it is currently unknown how much of the observed decrease in state prison populations can be attributed to decreases in the crime rate since its peak in the 1990s, as opposed to successful criminal sentencing reforms. This study uses new data on state sentencing reform policies in a decomposition of annual changes in state imprisonment rates between 1970 and 2019. Decreases in the property crime rate can account for 43%–60% of the observed decrease in the annual change in state imprisonment rates, whereas sentencing reforms account for another 12%–16%. Sentencing reforms have had their largest effects in the Midwest and South but have not contributed to decreases in the annual change in state imprisonment rates in the Northeast or West. These results uncover “varieties of decarceration” across the states and suggest that recent reform efforts—although effective—can only account for a portion of the observed decreases in state prison populations.