Prison population growth and crime reduction

The impact of state prison populations on crime is typically estimated by applying the lambda, the individual crime rate, of prisoners or arrestees. We outline the problems with this approach, attempt to reanalyze the widely divergent lambdas derived in past research, and make adjustments necessary to use lambdas for estimating the incapacitation impact. The result is an uncertain estimate of 16 to 25 index crimes averted per year per each additional prisoner. We argue that regression analysis can provide a better estimate of the impact of prison population growth. Applying the Granger test to pooled state data over 19 years, we found that prison population growth leads to lower crime rates but that crime rate changes have little or no short-term impact on prison population growth. Next we regressed crime rates on prison population and conclude that, on average, at least 17 index crimes are averted per additional prisoner. The impact is limited mainly to property crime.