The Deterrent Effect of Capital Punishment: Another View
In a recent paper in this Revieti*, Isaac Ehrlich found a negative relationship be-tueen the use o f the death penalty a n d homicide rates, over the three and a half decades from 1935 69 in the United States. This empirical finding, in apparent conflict with previous studies on the subject (see Thorsten Sellin, Robert Dann), takes on special meaning because (a) it is the first test of the capital punishment deterrence hypothesis to use econometric estimation techniques , and (b) its publication comes at a time when legislatures and the courts in both the United States a n d Canada are reshaping public policy on the use of the death penalty. Hence the importance of a second look a t Ehrlich's results. Such a reexamination, described below, suggests that the time-series model and data used by Ehrlich permit no inference about the deterrent effect of capital punishment on homicide. The data indicate that the parametersof Ehrlich's model are extremely sensitive to the choices of included explanatory variables and the functional form of the model; when recast in equally plausible alternative structures, the Ehrlich model fails to generate a deterrent effect for executions. Moreover, Ehrlich's methodo-logical approach to the question, estimation of a single structural equation relating executions to murders, sharply limits the policy implications of the estimated coefficients. Section I briefly describes Ehrlich's murder rate function and tests the model for parameter stability over the sample period. * C o l u m b ~ a Unirersit!. A n early rersion of this paper was wrltten before the Ehrlich appendix appeared: it uses an alternative data set. available on request from the authors. This paper was listed as a Columbia Workshop discuss~oli paper; it was published In a moditicd a n d less technical version in 1976. T h c conclusion a n d findings are similar despite the different data set. W e would like to thank Phoebus J. D h p m e s a n d David Krnnett for their comments a n d assistance. Section I1 considers his choice of explanatory variables and functional forms, and examines sensitivity of his estimate to these choices. Section 111 examines the pitfalls of inferring an execution-murder tradeoff from a single estimated equation. A representative sample of the empirical results reported by Ehrlich in support of the deterrence effect of capital punishment is his equation (I), Table 4, …