In 1976 the Supreme Court decision Gregg v. Georgia (428 U.S. 153) ended the 4-year moratorium on executions that had resulted from its 1972 decision in Furman v. Georgia (408 U.S. 238). In Furman the Court had ruled that the death penalty, as then administered in the United States, constituted cruel and unusual punishment in violation of the Eighth Amendment to the Constitution. Then, in Gregg, it had ruled that the death penalty is not, in all circumstances, cruel and unusual punishment, thereby opening the way for states to revise their capital punishment statutes to conform to the requirements of Gregg.
In the immediate aftermath of Gregg, a National Research Council report reviewed the evidence relating to the deterrent effect of the death penalty that had been published through the mid-1970s. That review was highly critical of the available research, concluding (1978, p. 9):
The flaws in the earlier analyses finding no effect and the sensitivity of the more recent analysis to minor variations in model specification and the serious temporal instability of the results lead the panel to conclude that available studies provide no useful evidence on the deterrent effect of capital punishment.
During the 35 years since Gregg, and particularly in the past decade, many studies have renewed the attempt to estimate the effect of capital punishment on homicide rates. Most researchers have used post-Gregg data from the United States to examine the statistical association between
homicide rates and the legal status or the actual implementation of the death penalty.
The studies have reached widely varying, even contradictory, conclusions, and commentary on the findings has sometimes been acrimonious. Some researchers have concluded that deterrent effects are large and robust across datasets and model specifications. For example, Dezhbakhsh, Rubin, and Shepherd (2003, p. 344) concluded that:
Our results suggest that capital punishment has a strong deterrent effect; each execution results, on average, in eighteen fewer murders with a margin of error of plus or minus ten. Tests show that results are not driven by tougher sentencing laws and are robust to many alternative specifications.
Similarly, Mocan and Gittings (2003, p. 453) stated the following:
The results show that each additional execution decreases homicides by about five, and each additional commutation increases homicides by the same amount, while an additional removal from death row generates one additional murder.
In 2004 testimony before Congress, Shepherd (2004, p. 1) summarized this line of evidence on the deterrent effect of capital punishment as follows:
Recent research on the relationship between capital punishment and crime has created a strong consensus among economists that capital punishment deters crime.
However, the claims that the evidence shows a substantial deterrent effect have been vigorously challenged. Kovandzic, Vieraitis, and Boots (2009, p. 803) concluded that:
Employing well-known econometric procedures for panel data analysis, our results provide no empirical support for the argument that the existence or application of the death penalty deters prospective offenders from committing homicide … policymakers should refrain from justifying its use by claiming that it is a deterrent to homicide and should consider less costly, more effective ways of addressing crime.
Others do not go so far as to claim that there is no deterrent effect, but instead argue that the findings supporting a deterrent effect are fragile, not robust. Donohue and Wolfers (2005, p. 794) reanalyzed several of the data sets used by the authors who claimed to have found robust deterrent effects and concluded that:
We find that the existing evidence for deterrence is surprisingly fragile, and even small changes in specifications yield dramatically different re-
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:
This study will assess the evidence on the deterrent effect of the death penalty—whether the threat of execution prevents homicides. The focus will be on studies completed since an earlier National Research Council assessment (National Research Council, 1978). A major objective of this study is to evaluate underlying reasons for the differing conclusions in more recent empirical studies about the effects of the legal status and actual practice of the death penalty on criminal homicide rates. The committee will develop a report about what can be concluded from these studies and also draw conclusions about the potential for future work to improve upon the quality of existing evidence.
Issues and questions to be examined include the following:
1. Does the available evidence provide a reasonable basis for drawing conclusions about the magnitude of capital punishment’s effect on homicide rates?
2. Are there differences among the extant analyses that provide a basis for resolving the differences in findings? Are the differences in findings due to inherent limitations in the data? Are there existing statistical methods and/or theoretical perspectives that have yet to be applied that can better address the deterrence question? Are the limitations of existing evidence reflective of a lack of information about the social, economic, and political underpinnings of homicide rates and/or the administration of capital punishment that first must be resolved before the deterrent effect of capital punishment can be determined?
3. Do potential remedies to shortcomings in the evidence on the deterrent effect of capital punishment have broader applicability for research on the deterrent effect of noncapital sanctions?
In addressing those questions, we focused on the studies that have been undertaken since the earlier assessment (National Research Council, 1978). That assessment has stood largely unchallenged: none of the recent work, whatever its conclusion regarding deterrence, relies on the earlier studies criticized in that report or attempts to rehabilitate the value of those studies.
It is important to make clear what is not in the committee’s charge. Deterrence is but one of many considerations relevant to deciding whether the death penalty is good public policy. Not all supporters of capital punishment base their argument on deterrent effects, and not all opponents would be affected by persuasive evidence of such effects. The case for capital punishment is sometimes based on normative retributive arguments that the death penalty is the only appropriate and proportional response to especially heinous crimes; the case against it is sometimes based on
similarly normative claims that the sanctity of human life precludes state-sanctioned killings, regardless of any possible social benefits of capital punishment. Separate from normative considerations, deterrence is not the only empirical issue relevant to the debate over capital punishment. Other considerations include whether capital punishment can be administered in a nondiscriminatory and consistent fashion, whether the risk of a mistaken execution of an innocent person is acceptably small, and the cost of administering the death penalty in comparison with other sanction alternatives.
Although there is empirical evidence on the issues of discrimination, mistakes, and cost, the charge to the committee does not include these questions. Nor have we been charged with rendering an overall judgment on whether capital punishment is good public policy. We have been tasked only with assessing the scientific quality of the post-Gregg evidence on the deterrent effect of capital punishment and making recommendations for improving the scientific quality and policy relevance of future research.
In including recommendations for future research, the study’s statement of task recognized that potential remedies to shortcomings in the evidence on the deterrent effect of capital punishment on homicide might also be used in the study of the crime prevention effects of noncapital sanctions. Thus, this report also offers recommendations for improving the scientific quality and policy relevance of that research.
The post-Gregg studies can be divided into two types on the basis of the type of data analyzed. Panel data studies analyze sets of states or counties measured over time, usually from about 1970 to 2000. These studies relate homicide rates over time and the jurisdictions covered to the legal status of capital punishment or the frequency of executions or both. Time-series studies generally cover only a single geographic unit, which may be as large as a nation or as small as a city. These studies usually examine whether there are short-term changes in homicide rates in that geographic unit in the aftermath of an execution. We review and critique these two types of studies separately because their design and statistical methods are quite different.
Assessing the deterrent effect of the death penalty is much more than a question of interest to social science research. It is a matter of importance to U.S. society at large, and we expect that a potentially broad audience will want to understand how the committee reached its conclusions. Yet the research that the committee has had to appraise is a body of formal empirical work that makes use of highly technical concepts and techniques. The committee has been mindful of the importance of reaching as broad an audience as possible while meeting the fundamental requirement that the report be scientifically grounded. With this in mind, Chapters 1, 2, and 3 (as well as the summary) have been written for a broad, largely policy audience, largely avoiding technical language. In contrast, Chapters 4 and
5 include some exposition and analyses that are aimed for the researchers in the field.
Chapter 2 summarizes homicide rates and the legal status and practice of execution in the United States from 1950 to the present. Chapter 3 provides an overview of the possible mechanisms by which the legal status and practice of execution might affect homicide rates and also provides a nontechnical primer on some of the key challenges to making valid inferences about the deterrent effect of the death penalty. Chapters 4 and 5 review and assess the panel and time-series studies, respectively. Chapter 6 elaborates on the theoretical and statistical challenges to drawing valid conclusions about the deterrent effect of the death penalty, and presents our conclusions and recommendations for future research.
Berk, R. (2005). New claims about executions and general deterrence: Déjà vu all over again? Journal of Empirical Legal Studies, 2(2), 303-330.
Cochran, J.K., and Chamlin, M.B. (2000). Deterrence and brutalization: The dual effects of executions. Justice Quarterly, 17(4), 685-706.
Dezhbakhsh, H., Rubin, P.H., and Shepherd, J.M. (2003). Does capital punishment have a deterrent effect? New evidence from postmoratorium panel data. American Law and Economics Review, 5(2), 344-376.
Donohue, J.J., and Wolfers, J. (2005). Uses and abuses of empirical evidence in the death penalty debate. Stanford Law Review, 58(3), 791-845.
Donohue, J.J., and Wolfers, J. (2009). Estimating the impact of the death penalty on murder. American Law and Economics Review, 11(2), 249-309.
Kovandzic, T.V., Vieraitis, L.M., and Boots, D.P. (2009). Does the death penalty save lives? Criminology & Public Policy, 8(4), 803-843.
Mocan, H.N., and Gittings, R.K. (2003). Getting off death row: Commuted sentences and the deterrent effect of capital punishment. Journal of Law & Economics, 46(2), 453-478.
National Research Council. (1978). Deterrence and Incapacitation: Estimating the Effects of Criminal Sanctions on Crime Rates. Panel on Research on Deterrent and Incapacitative Effects, A. Blumstein, J. Cohen, and D. Nagin (Eds.). Committee on Research on Law Enforcement and Criminal Justice. Assembly of Behavioral and Social Sciences. Washington, DC: National Academy Press.
Rubin, P.H. (2009). Don’t scrap the death penalty. Criminology & Public Policy, 8(4), 853-859.
Shepherd, J.M. (2004). Testimony on Crime and Deterrence: Hearing on H.R. 2934, the Terrorist Penalties Enhancement Act of 2003. Subcommittee on Crime, Terrorism, and Homeland Security, House Judiciary Committee. Available: http://judiciary.house.gov/legacy/shepherd042104.pdf [January 2012].
Shepherd, J.M. (2005). Deterrence versus brutalization: Capital punishment’s differing impacts among states. Michigan Law Review, 104(2), 203-255.
Thomson, E. (1999). Effects of an execution on homicides in California. Homicide Studies, 3(2), 129-150.
Zimmerman, P.R. (2009). Statistical variability and the deterrent effect of the death penalty. American Law and Economics Review, 11(2), 370-398.