Do execution moratoriums increase homicide? Re-examining evidence from Illinois

This article revisits the event study by Cloninger and Marchesini (2006), who find that the declaration of the Illinois’ death penalty moratorium on 31 January 2000 had a homicide-promoting effect and resulted in 150 additional homicides over the period 2000–2003. We reassess the author’s identification strategy, which they refer to as ‘portfolio approach’ and which draws upon event studies in finance research. We argue that their methodology is not applicable in crime studies. Instead, we apply univariate time-series methods to test for a structural break at a known and unknown break date. We allow for unknown break points as the structural break might have occurred slightly earlier (criminals might have anticipated the moratorium) or later (due to persistence in criminal behaviour). In addition, we implement the synthetic control estimator which approximates the counterfactual homicide series by a weighted average of homicide outcomes in other US states. Based on various testing methods and two distinct data sets, we conclude that there is no empirical evidence to support the hypothesis that the Illinois’ execution moratorium significantly increased homicides.