The Deterrent Effect of Capital Punishment: Evidence from a “Judicial Experiment”

I. INTRODUCTION The contemporary debate over capital punishment involves a number of important arguments based on either moral principles or social welfare considerations. The primary social welfare issue, viewed as "the most important single consideration for both sides in the death penalty controversy," is whether capital punishment deters capital crimes; see, for example, Zimring and Hawkins (1986). This issue is also of great interest to several states that are currently considering a change in their death penalty laws. (1) Psychologists and criminologists studied the death penalty initially and reported no deterrent effect; see, for example, Sellin (1959) and Eysenck (1970). Economists joined the debate later, with Ehrlich's 1975 and 1977 studies that report a significant deterrent effect applying regression analysis to U.S. aggregate data for 1933-69 and state-level data for 1940 and 1950. Ensuing studies use either Ehrlich's data with different regression specifications--different regressors, functional form, or endogenous variables--or postmoratorium data with a variant of Ehrlich's regression model. Results range from a substantial deterrent effect to no effect or a small adverse effect. (2) This study uses a new approach to advance the deterrence literature. We exploit an important characteristic that other studies have not considered: the experimental nature of the moratorium that the Supreme Court imposed on executions during the 1970s. The moratorium can be viewed as an exogenously imposed "judicial experiment." The experiment's effect on murder rates provides evidence about the deterrent effect of capital punishment. We compare the murder rate for each state immediately before and after it suspended or reinstated the death penalty. Many factors that affect crime--for example, law enforcement, judicial, demographic, and economic variables--change only slightly over a short period of time. Therefore, changes in a state's murder rate quickly following a change in its death penalty law can be attributed to the legal change. Also, there are considerable cross-state variations in the timing and duration of the moratorium that began and ended in different years in different states, ranging from four to thirty years (see Tables 1 and 2). (3) These variations can strengthen our comparison-based inference, because observing similar changes in murder rates immediately after the same legal change in different years and in various states provides compelling evidence of the moratorium's effect on murder. Moreover, our analysis benefits from the experiences of the states that suspended and reinstated their death penalty statutes several times. (4) For states that experience similar murder rate changes after suspensions (or reinstatements) in different crime, economic, and demographic environments, the contribution of the legal change to murder is paramount. We supplement the before-and-after comparisons with time-series and panel data regression analyses. Unlike most existing studies, we use both pre- and postmoratorium data. The regressions disentangle the effect of having death penalty laws on murder from the effect of actual executions on murder. Basing our inference on two different approaches allows us to corroborate our findings and also detract from the modeling concerns voiced at previous studies that use only regression analysis. We also apply a battery of tests to examine the robustness of the findings. These include regression sensitivity checks with 89 additional regression equations that differ from our base models in regressors, functional form, data, and estimation method. In addition, we test whether the moratorium has an effect on property crimes that are not punishable by death. If the relationship between the moratorium and these crimes is similar to the relationship between the moratorium and murder, then we should suspect that murder rate changes are the result of broader trends in criminal behavior affecting all crimes. …