Using Sentence Enhancements to Distinguish between Deterrence and Incapacitation

Abstract

Differentiating empirically between deterrence and incapacitation is difficult since both are a function of expected punishment. In this paper we demonstrate that the introduction of sentence enhancements (i.e., increased punishments that are added on to prison sentences that would have been served anyway) provides a direct means of measuring deterrence. Because the criminal would have been sentenced to prison anyway, there is no additional incapacitation effect from the sentence enhancement in the short-run. Therefore, any immediate decrease in crime must be due to deterrence. We test the model using California's Proposition 8, which imposed sentence enhancements for a selected group of crimes. Using a differences-in-differences approach (comparing changes in eligible crimes relative to non-eligible crimes in California and the rest of the nation), Proposition 8 appears to reduce eligible crimes by four percent in the year following its passage and eight percent three years after passage. These immediate effects are consistent with deterrence. The impact of the law continues to increase five to seven years after its passage, suggesting that incapacitation may be important as well.