Homicide and Deterrence: A Reexamination of the United States Time-Series Evidence*

Since the publication of Isaac Ehrlich's controversial articles [6; 9] on murder and capital punishment, there has been extensive criticism of his findings. Using regression analysis with U.S. time-series data, Ehrlich found that increases in the relative frequency of arrest, the relative frequency of conviction given arrest and the relative frequency of execution given conviction, reduce the number of homicides per capita and that the magnitudes of these effects are consistent with sharp predictions-the "elasticity conditions"derived from the hypothesis that potential murderers act as if they were maximizing expected utility.' Among the criticisms of Ehrlich's time-series work on homicide, the most important are: (1) The FBI data used by Ehrlich to measure homicides and the probabilities of punishment is highly suspect, especially during the 1930s [3]. (2) Ehrlich's results are sensitive to the inclusion of additional explanatory variables and the choice of functional form [3; 19; 26]. (3) Ehrlich's regressions are unstable over the 1960s [26]. (4) The negative correlations between the homicide rate and the probabilities of punishment found by Ehrlich may be explained by the effect of the homicide rate on the probabilities of punishment not vice versa [17]. Ehrlich has responded to these criticisms in some detail [7; 8; 9; 11]; perhaps his strongest rebuttal is his cross-sectional study of homicide [9] which replicated the deterrence findings reported in his 1975 time-series article. Recently, however, Ehrlich's crosssectional results have also come under fire. McManus [25] and Leamer [22; 23] have argued that Ehrlich's cross-sectional finding that capital punishment is a deterrent to homicide is sensitive to one's prior beliefs concerning the variables to be included in the homicide regression. The purpose of this paper is not to debate the merits of Ehrlich's research but to move the debate forward by presenting new updated time-series estimates of the U.S. homicide function. Within the context of the new estimates presented in this paper, the criticisms of