sults. Our key insight is that the death penalty—at least as it has been implemented in the United States since Gregg ended the moratorium on executions—is applied so rarely that the number of homicides it can plausibly have caused or deterred cannot be reliably disentangled from the large year-to year changes in the homicide rate caused by other factors.
Berk (2005, p. 328) reached a similar conclusion:
… the results raise serious questions about whether anything useful about the deterrent value of the death penalty can ever be learned from an observational study with the data that are likely to be available.
Not surprisingly, the criticisms of the research claiming to have found deterrent effects have generated defenses of the research findings and the methodologies used, as well as counterclaims about the deficiencies in the methods used by the critics. For instance, in response to the Kovandzic, Vieraitis, and Boots (2009) claim of no deterrent effect, Rubin (2009, p. 858) argued that:
the weight of the evidence as well as the theoretical predictions both argue for deterrence, and econometrically flawed studies such as this article are insufficient to overthrow this presumption.
In response to Donohue and Wolfers (2005, 2009), Zimmerman (2009, p. 396) argued that:
This paper shows that many of D&W’s [Donohue and Wolfers] criticisms of Zimmerman’s original work do not hold up under scrutiny, and other authors have also rebutted D&W’s criticisms of their research.
Beyond disagreement about whether the research evidence shows a deterrent effect of capital punishment, some researchers claim to have found a brutalization effect from state-sanctioned executions such that capital punishment actually increases homicide rates (see, e.g., Cochran and Chamlin, 2000; Thomson, 1999). Evidence in support of a brutalization effect is mostly the work of sociologists, but it is notable that in her latter work Shepherd also concluded that brutalization effects may be present (Shepherd, 2005).
The Committee on Deterrence and the Death Penalty was organized against this backdrop of conflicting claims about the effect of capital punishment on homicide rates, with the following charge: