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 ar restees. We outline the problems with this approach, attempt to reanal yze the widely divergent lambdas derived in past research, and make ad justments necessary to use lambdas for estimating the incapacitation i mpact. The result is an uncertain estimate of 16 to 25 index crimes av erted per year per each additional prisoner. We argue that regression analysis can provide a better estimate of the impact of prison populat ion growth. Applying the Granger test to pooled state data over 19 yea rs, we found that prison population growth leads to lower crime rates but that crime mte changes have little or no short-term impact on pris on population growth. Next we regressed crime rates on prison populati on and conclude that, on average, at least 17 index crimes are averted per additional prisoner. The impact is limited mainly to property cri me.