Deterrence and the Death Penalty

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INSTITUTE OF MEDICINE

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PAPERBACK (2012)

Deterrence and the Death Penalty

Daniel S. Nagin and John V. Pepper, editors; Committee on Deterrence

and the Death Penalty; Committee on Law and Justice; Division on

Behavioral and Social Sciences and Education; National Research Council

Deterrence and the Death Penalty

DETERRENCE

AND THE

DEATH PENALT Y

Committee on Deterrence and the Death Penalty

Daniel S. Nagin and John V. Pepper, Editors

Committee on Law and Justice

Division of Behavioral and Social Sciences and Education

Deterrence and the Death Penalty

THE NATIONAL ACADEMIES PRESS • 500 Fifth Street, NW • Washington, DC 20001

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Suggested citation: National Research Council. (2012). Deterrence and the Death

Penalty. Committee on Deterrence and the Death Penalty, Daniel S. Nagin and John

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Deterrence and the Death Penalty

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Deterrence and the Death Penalty

COMMITTEE ON DETERRENCE AND THE DEATH PENALTY

DANIEL S. NAGIN (Chair), H. John Heinz III College, Carnegie Mellon

University

KERWIN K. CHARLES, Harris School of Public Policy Studies,

University of Chicago

PHILIP J. COOK, Sanford School of Public Policy, Duke University

STEVEN N. DURLAUF, Department of Economics, University of

Wisconsin–Madison

AMELIA M. HAVILAND, H. John Heinz III College, Carnegie Mellon

University

GERARD E. LYNCH, U.S. Court of Appeals for the Second Circuit

CHARLES F. MANSKI, Department of Economics, Northwestern

University

JAMES Q. WILSON, School of Public Policy, Pepperdine University, and

Clough Center for the Study of Constitutional Democracy, Boston

College

JANE L. ROSS, Study Director

JOHN V. PEPPER, Consultant

KEIKO ONO, Senior Program Associate

CAROL HAYES, Christine Mirzayan Fellow

BARBARA BOYD, Administrative Associate

Deterrence and the Death Penalty

COMMITTEE ON LAW AND JUSTICE

2012

JEREMY TRAVIS (Chair), John Jay College of Criminal Justice, City

University of New York

CARL C. BELL, Community Mental Health Council, Inc., Chicago, IL

JOHN J. DONOHUE, III, Stanford Law School, Stanford University

MARK A.R. KLEIMAN, Department of Public Policy, University of

California, Los Angeles

GARY LAFREE, Department of Criminology and Criminal Justice,

University of Maryland

JANET L. LAURITSEN, Department of Criminology and Criminal

Justice, University of Missouri-St. Louis

GLENN C. LOURY, Department of Economics, Brown University

CHARLES F. MANSKI, Department of Economics, Northwestern

University

TERRIE E. MOFFITT, Department of Psychology and Neuroscience,

Duke University

DANIEL S. NAGIN, H. John Heinz III College, Carnegie Mellon

University

RUTH D. PETERSON, Criminal Justice Research Center, Ohio State

University

ANNE MORRISON PIEHL, Department of Economics and Program in

Criminal Justice, Rutgers University

DANIEL B. PRIETO, Public Sector Strategy & Innovation, IBM Global

Business Services, Washington, DC

ROBERT J. SAMPSON, Department of Sociology, Harvard University

DAVID WEISBURD, Department of Criminology, Law and Society,

George Mason University

CATHY SPATZ WIDOM, Psychology Department, John Jay College of

Criminal Justice, City University of New York

PAUL K. WORMELI, Integrated Justice Information Systems,

Ashburn, VA

JANE L. ROSS, Director

BARBARA BOYD, Administrative Associate

Deterrence and the Death Penalty

vii

IN MEMORIAM

James Q. Wilson

1931-2012

“I’ve tried to follow the facts wherever they land.”

This report is dedicated to James Q. Wilson for

his long service to the National Research Council,

his inﬂuential career of scholarship and public

service, and his unblinking commitment to the

principle that science requires us to interpret

the evidence as it is, not as we want it to be.

Deterrence and the Death Penalty

Preface

ore than three decades ago, in Deterrence and Incapacitation:

Estimating the Effects of Criminal Sanctions on Crime Rates,

the National Research Council (NRC) (1978, p. 9) concluded

that “available studies provide no useful evidence on the deterrent effect

of capital punishment.” That report was issued 2 years after the Supreme

Court decision in Gregg v. Georgia ended a 4-year moratorium on execu-

tion in the United States. In the 35 years since the publication of that report,

especially in recent years, a considerable number of post-Gregg studies have

attempted to estimate the effect of the legal status or the actual implemen-

tation of the death penalty on homicide rates. Those studies have reached

widely varying conclusions.

Against this background, the NRC formed the Committee on Deter-

rence and the Death Penalty to address whether the available evidence

provides a reasonable basis for drawing conclusions about the magnitude of

the effect of capital punishment on homicide rates. At a workshop on April

28-29, 2011, workshop papers commissioned by the committee (which will

be published in a special issue of the Journal of Quantitative Criminology)

were presented and discussed by their authors: Robert J. Apel, University at

Albany, State University of New York; Aaron Chalﬁn, University of Califor-

nia, Berkeley; Chao Fu, University of Wisconsin–Madison; Justin McCrary,

University of California, Berkeley; Salvador Navarro, University of Western

Ontario, Ontario, Canada; John V. Pepper, University of Virginia; and

Steven Raphael, University of California, Berkeley. The workshop also

included comments on the presentations by Jeffrey Grogger, University

of Chicago; Guido Imbens, Harvard University; Kenneth C. Land, Duke

Deterrence and the Death Penalty

x PREFACE

University; Christopher Sims, Princeton University; and Justin Wolfers,

University of Pennsylvania.

The committee appreciates the contributions of these presenters and

those who commented on them to the development of its report. In ad-

dition, John V. Pepper provided invaluable assistance to the committee

throughout its deliberations. The work of staff members from the Com-

mittee on Law and Justice of the NRC facilitated the committee’s work in

many ways. Thanks are due to Jane L. Ross, study director; Keiko Ono,

senior program associate; Carol Hayes, Christine Mirzayan fellow; and

Barbara Boyd, administrative coordinator.

Many individuals at the NRC assisted the committee. We thank Kirsten

Sampson-Snyder, who shepherded the report through the NRC review pro-

cess, Eugenia Grohman, who edited the draft report, and Yvonne Wise, for

processing the report through ﬁnal production.

This report has been reviewed in draft form by individuals chosen for

their diverse perspectives and technical expertise, in accordance with pro-

cedures approved by the NRC’s Report Review Committee. The purpose

of this independent review is to provide candid and critical comments that

will assist the institution in making its published report as sound as possible

and to ensure that the report meets institutional standards for objectivity,

evidence, and responsiveness to the study charge. The review comments

and draft manuscript remain conﬁdential to protect the integrity of the

deliberative process. We thank the following individuals for their review of

this report: John Donohue, III, Stanford Law School, Stanford University;

Andrew Gelman, Department of Statistics and Department of Political Sci-

ence, Columbia University; Kenneth C. Land, Department of Sociology,

Duke University; Candice Odgers, School of Social Ecology, University of

California, Irvine; Ricardo Reis, Department of Economics, Columbia Uni-

versity; Greg Ridgeway, RAND Safety and Justice Program, RAND Center

on Quality Policing, RAND Corporation; Robert J. Sampson, Department

of Sociology, Harvard University; Dick Thornburgh, Counsel, K&L Gates,

LLP, and former Attorney General of the United States; Petra E. Todd,

Department of Economics, University of Pennsylvania; and Michael Tonry,

School of Law, University of Minnesota, Minneapolis.

Although the reviewers listed above have provided many constructive

comments and suggestions, they were not asked to endorse the conclusions

or recommendations nor did they see the ﬁnal draft of the report before its

release. The review of this report was overseen by Gary LaFree, National

Consortium for the Study of Terrorism and Responses to Terrorism, Univer-

sity of Maryland, and John T. Monahan, University of Virginia Law School.

Appointed by the NRC, they were responsible for making certain that an

independent examination of this report was carried out in accordance with

institutional procedures and that all review comments were carefully con-

Deterrence and the Death Penalty

PREFACE xi

sidered. Responsibility for the ﬁnal content of this report rests entirely with

the authoring committee and the institution.

This report is dedicated to James Q. Wilson. Jim was a valued member

of this and many other NRC committees on which he served over his long

and inﬂuential career. Jim’s contributions to scholarship and public service

will stand as enduring testimony to the power of his intellect. He was a

quiet but forceful proponent for balanced and clear-minded assessment of

the evidence. I ﬁrst met Jim in my role as a staff member of the 1978 NRC

committee that resulted in report Deterrence and Incapacitation: Estimat-

ing the Effect of Criminal Sanctions on Crime Rates. I was deeply impressed

by the clarity of his thought and gift for communication. He served as a

role model for me ever since. I was thus especially honored that he agreed

to serve on this committee, which was greatly aided by his constructive

participation throughout our deliberations.

Daniel S. Nagin, Chair

Committee on Deterrence and the Death Penalty

Deterrence and the Death Penalty

xiii

Contents

SUMMARY 1

Shortcomings in Existing Research, 4

Speciﬁcation of the Sanction Regime for Homicide, 4

Potential Murderers’ Perceptions of and Responses to

Capital Punishment, 5

Strong and Unveriﬁable Assumptions, 6

Next Steps for Research, 7

References, 8

1 INTRODUCTION 9

The Current Debate, 9

Committee Charge and Scope of Work, 11

References, 14

2 CAPITAL PUNISHMENT IN THE POST-GREGG ERA 15

Executions and Death Sentences Over Time, 15

Use of the Death Penalty, 20

References, 26

3 DETERMINING THE DETERRENT EFFECT OF

CAPITAL PUNISHMENT: KEY ISSUES 27

Concepts of Deterrence, 28

Sanction Regimes, 32

Deterrence and the Death Penalty

xiv CONTENTS

Data Issues, 36

Variations in Murder Rates, 37

Reciprocal Effects Between Homicide Rates and

Sanction Regimes, 41

Summary, 43

References, 44

4 PANEL STUDIES 47

Panel Studies Reviewed, 48

Methods Used: Overview, 48

The Studies, Their Characteristics, and the Effects Found, 49

Specifying the Expected Cost of Committing a Capital

Homicide: f(Z

), 54

Model Assumptions, 63

Beneﬁts of Random Assignment, 64

Fixed Effect Regression Model, 65

Instrumental Variables, 66

Homogeneity, 68

Conclusion, 70

References, 71

5 TIME-SERIES STUDIES 75

Basic Conceptual Issues, 76

Execution Event Studies, 76

Studies of Deviations from Fitted Trends, 78

Vector Autoregressions, 82

Evidence Under Existing Criminal Sanction Regimes, 82

Granger Causality and Causality as Treatment Response, 86

Choice of Variables in VAR Studies, 88

Inferences Under Alternative Sanction Regimes, 89

Event Studies, 90

Time-Series Regressions, 92

Cross-Polity Comparisons, 94

Conclusions, 97

References, 99

6 CHALLENGES TO IDENTIFYING DETERRENT EFFECTS 101

Data on Sanction Regimes, 104

Perceptions of Sanction Risks, 105

Measurement of Perceptions, 107

Inference on Perceptions from Homicide Rates Following

Executions, 110

Deterrence and the Death Penalty

CONTENTS xv

Identifying Effects: Feedbacks and Unobserved Confounders, 111

Feedback Effects, 111

Omitted Variables, 112

The Equilibrium Effect, 113

Addressing Model Uncertainty with Weaker Assumptions, 115

Model Averaging, 116

Partial Identiﬁcation, 119

References, 121

Appendix: Biographical Sketches of Committee Members and Staff 125

Deterrence and the Death Penalty

Summary

n 1976, the Supreme Court decision in 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 the immediate

aftermath of Gregg, an earlier report of the National Research Council

(NRC) reviewed the evidence relating to the deterrent effect of the death

penalty that had been gathered through the mid-1970s. That review was

highly critical of the earlier research and concluded (National Research

Council, 1978, p. 9) 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 additional 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, the actual implementation of

the death penalty, or both. The studies have reached widely varying, even

contradictory, conclusions. Some studies conclude that executions save

large numbers of lives; others conclude that executions actually increase

homicides; and still others conclude that executions have no effect on

homicide rate. Commentary on the scientiﬁc validity of the ﬁndings has

sometimes been acrimonious. The Committee on Deterrence and the Death

Penalty was convened against this backdrop of conﬂicting claims about the

effect of capital punishment on homicide rates. The committee addressed

three main questions laid out in its charge:

Deterrence and the Death Penalty

2 DETERRENCE AND THE DEATH PENALTY

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 ba-

sis 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 homi-

cide 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 de-

terrent effect of capital punishment have broader applicability for

research on the deterrent effect of noncapital sanctions?

CONCLUSION AND RECOMMENDATION: The committee con-

cludes that research to date on the effect of capital punishment on ho-

micide is not informative about whether capital punishment decreases,

increases, or has no effect on homicide rates. Therefore, the committee

recommends that these studies not be used to inform deliberations

requiring judgments about the effect of the death penalty on homicide.

Consequently, claims that research demonstrates that capital punish-

ment decreases or increases the homicide rate by a specied amount

or has no effect on the homicide rate should not inuence policy judg-

ments about capital punishment.

The committee was disappointed to reach the conclusion that research

conducted in the 30 years since the earlier NRC report has not sufﬁciently

advanced knowledge to allow a conclusion, however qualiﬁed, about the ef-

fect of the death penalty on homicide rates. Yet this is our conclusion. Some

studies play the useful role, either intentionally or not, of demonstrating the

fragility of claims to have or not to have found deterrent effects. However,

even these studies suffer from two intrinsic shortcomings that severely limit

what can be learned from them about the effect of the death penalty—as it

has actually been administered in the United States in the past 35 years—on

the death penalty.

Properly understood, the relevant question about the deterrent effect of

capital punishment is the differential or marginal deterrent effect of execu-

tion over the deterrent effect of other available or commonly used penalties,

speciﬁcally, a lengthy prison sentence or one of life without the possibility of

Deterrence and the Death Penalty

SUMMARY 3

parole. One major deﬁciency in all the existing studies is that none specify

the noncapital sanction components of the sanction regime for the punish-

ment of homicide. Another major deﬁciency is the use of incomplete or

implausible models of potential murderers’ perceptions of and response to

the capital punishment component of a sanction regime. Without this basic

information, it is impossible to draw credible ﬁndings about the effect of

the death penalty on homicide.

Commentary on research ﬁndings often pits studies claiming to ﬁnd

statistically signiﬁcant deterrent effects against those ﬁnding no statistically

signiﬁcant effects, with the latter studies sometimes interpreted as imply-

ing that there is no deterrent effect. A fundamental point of logic about

hypothesis testing is that failure to reject a null hypothesis does not imply

that the null hypothesis is correct.

Our mandate was not to assess whether competing hypotheses about

the existence of marginal deterrence from capital punishment are plausible,

but simply to assess whether the empirical studies that we have reviewed

provide scientiﬁcally valid evidence. In its deliberations and in this report,

the committee has made a concerted effort not to approach this question

with a prior assumption about deterrence. Having reviewed the research

that purports to provide useful evidence for or against the hypothesis that

the death penalty affects homicide rates, we conclude that it does not pro-

vide such evidence.

A lack of evidence is not evidence for or against the hypothesis. Hence,

the committee does not construe its conclusion that the existing studies are

uninformative as favoring one side or the other side in the long-standing

debate about deterrence and the death penalty. The committee also empha-

sizes that deterrence is but one of many considerations relevant to rendering

a judgment on whether the death penalty is good public policy.

Even though the scholarly evidence on the deterrent effect of capital

punishment is too weak to guide decisions, this does not mean that people

should have no views on capital punishment. Judgment about whether

there is a deterrent effect is still relevant to policy, but that judgment

should not be justiﬁed based on evidence from existing research on capital

punishment’s effect on homicide. Just as important, the committee did not

investigate the moral arguments for or against capital punishment or the

empirical evidence on whether capital punishment is administered in a

nondiscriminatory and consistent fashion. Nor did it investigate whether

the risk of mistaken execution is acceptably small or how the cost of ad-

ministering the death penalty compares to other sanction alternatives. All

of these issues are relevant to making a judgment about whether the death

penalty is good public policy.

Our charge was also limited to assessing the evidence on the deterrent

effect of the death penalty on murder, not the deterrent effect of noncapital

Deterrence and the Death Penalty

4 DETERRENCE AND THE DEATH PENALTY

sanctions on crime more generally. Our negative conclusion on the infor-

mativeness of the evidence on the former issue should not be construed as

extending to the latter issue because the committee did not review the very

large body of evidence on the deterrent effect of noncapital sanctions.

SHORTCOMINGS IN EXISTING RESEARCH

The post-Gregg studies are usefully divided into two categories based

on the type of data analyzed. One category, which we call panel data stud-

ies, analyzes sets of states or counties measured over time, usually from

about 1970 to 2000. These studies relate homicide rates to variations over

time and across states or counties in the legal status of capital punishment

and/or the frequency of executions. The second category, which we call

time-series studies, generally studies only a single geographic unit. The geo-

graphic unit 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.

As noted above, research on the effect of capital punishment on ho-

micide suffers from two fundamental ﬂaws that make them uninformative

about the effect of capital punishment on homicide rates: they do not

specify the noncapital sanction components of the sanction regime for the

punishment of homicide, and they use incomplete or implausible models of

potential murderers’ perceptions of and response to the capital punishment

component of a sanction regime. In addition, the existing studies use strong

and unveriﬁable assumptions to identify the effects of capital punishment

on homicides.

Specication of the Sanction Regime for Homicide

The sanction regime for homicide comprises both the capital and non-

capital sanctioning options that are available for its punishment and the

policies governing the administration of these options. The relevant ques-

tion regarding the deterrent effect of capital punishment is the differential

deterrent effect of execution in comparison with the deterrent effect of other

available or commonly used penalties. We emphasize “differential” because

it is important to recognize that even in states that make the most intense

use of capital punishment, most convicted murderers are not sentenced to

death but to a lengthy prison sentence—often life without the possibility

of parole.

None of the studies that we reviewed (both those using a panel ap-

proach and those using time-series approaches) accounted for the severity

of noncapital sanctions in their analyses. As discussed in Chapters 4 and 6,

there are sound reasons to expect that the severity of the noncapital sanc-

Deterrence and the Death Penalty

SUMMARY 5

tions for homicide varies systematically with the availability of capital pun-

ishment, the intensity of use of capital punishment, or both. For example,

the political culture of a state may affect the frequency of the use of capital

punishment and also the severity of noncapital sanctions for homicide.

Thus, any effect that these noncapital sanctions have on the homicide rate

may contaminate any estimated effect of capital punishment.

Potential Murderers’ Perceptions of and Responses to Capital Punishment

A by-product of the absence of consideration of the noncapital com-

ponent of the sanction regime is that no studies consider how the capital

and noncapital components of a regime combine in affecting the behavior

of potential murderers. Only the capital component of the sanction regime

has been studied, and this in itself shows both a serious conceptual ﬂaw

and a serious data ﬂaw in the entire body of research.

Several factors make the attempts by the panel studies to specify the

capital component of state sanctions regimes uninterpretable. First, the

ﬁndings are very sensitive to the way the risk of execution is speciﬁed. Sec-

ond, there is no logical basis for resolving disagreements about how this

risk should be measured.

Much of the panel research simply assumes that potential murderers

respond to the objective risk of execution. There are signiﬁcant complexities

in computing this risk even for a well-informed researcher, let alone for a

potential murderer. Among these complexities are that only 15 percent of

people who have been sentenced to death since 1976 have actually been

executed and a large fraction of death sentences are subsequently reversed.

None of the measures that are used in the research have been shown to be

a better measure of the risk of execution than any others. Thus, even if one

assumes that a potential murderer’s perceived risk corresponds to the actual

risk, there is no basis for arbitrating the competing claims about what is

the “right” risk measure.

The committee is also skeptical that potential murderers can possibly

estimate the objective risk, whatever it is. Hence, there is good reason to be-

lieve that perceived risk deviates from the objective risk. The research does

not address how potential murderers’ perceptions of capital punishment—

and, more generally, noncapital sanction risks—are formed.

The time-series studies come in many forms—studies of a single ex-

ecution event, studies of many events, and studies with a cross-polity

dimension—but a common feature of the studies is that none of them at-

tempts to specify even the capital component of the overall sanction regime.

This is a crucial shortcoming and is exempliﬁed in the time-series analyses

that examine the association between deviations of number of executions

from a ﬁtted trend line and deviations of homicides from a ﬁtted trend line.

Deterrence and the Death Penalty

6 DETERRENCE AND THE DEATH PENALTY

For potential murderers to possibly be responsive to deviations from the

execution trend line, they have to be attentive to it. The studies are silent on

two key questions: (1) Why are potential murderers attentive to the trend

line in the number of executions? (2) Why do they respond to deviations

from the trend line?

If time-series analyses ﬁnd that homicide rates are not responsive to

such deviations, it may be that potential murderers are responding to the

trend line in executions but not to deviations from it. For example, a ris-

ing trend in the number of executions might be perceived as signaling a

toughening of the sanction regime, which might deter potential murderers.

Alternatively, if a time-series analysis ﬁnds that homicide rates are respon-

sive to such deviations, the question is why? One possibility is that potential

murderers interpret the deviations as new information about the intensity

of the application of capital punishment—that is, they perceive a change

in the part of the sanction regime relating to application of capital punish-

ment. If so, a deviation from the execution trend line may cause potential

murderers to alter their perceptions of the future course of the trend line,

which in turn may change their behavior.

Yet, even accepting this idea, a basic question persists. Why should the

trend lines ﬁt by researchers coincide with the perceptions of potential mur-

derers about trends in executions? Because there are no studies that include

empirical analyses on the question of how potential murderers perceive the

risk of sanctions, there is no basis for assuming that the trend line speciﬁed

by researchers corresponds to the trend line (if any) that is perceived by

potential murderers. If researchers and potential murderers do not perceive

trends the same way, then time-series analyses do not correctly identify

what potential murderers perceive as deviations. Because of this basic ﬂaw

in the research, the committee has no basis for assessing whether the ﬁnd-

ings of time-series studies reﬂect a real effect of executions on homicides

or are artifacts of models that incorrectly specify how deviations from a

trend line cause potential murderers to update their forecasts of the future

course of executions.

Strong and Unveriable Assumptions

To obtain a single estimate that speciﬁes the effect of capital punish-

ment on homicide, researchers invariably rely on a range of strong and

unveriﬁed assumptions. In part (as discussed above), this reﬂects the lack of

basic information on the relevant sanction regimes for homicide and the as-

sociated perceptions of risk. None of the studies accounts for the noncapital

component of the sanction regime, and potential murderers’ risk percep-

tions are assumed to depend on observable frequencies of arrest, conviction,

and execution. The ad hoc choices of alternative models of risk perceptions

Deterrence and the Death Penalty

SUMMARY 7

lead to very different inferences on the effects of capital punishment, and

none of them is inherently any more justiﬁable than any other.

Additional data and research on sanction regimes and risk perceptions

may serve to reduce this form of model uncertainty. However, even if these

uncertainties are fully reconciled, a more fundamental problem is that the

outcomes of counterfactual sanction policies are unobservable. That is,

there is no way to determine what would have occurred if a given state

had a different sanction regime. In light of this observational problem, the

available data cannot reveal the effect of capital punishment itself since the

policy-relevant question is whether capital punishment deters homicides

relative to other sanction regimes. That is, the data alone cannot reveal

what the homicide rate in a state without (with) a capital punishment re-

gime would have been had the state (not) had such a regime.

The standard procedure in capital punishment research has been to

impose sufﬁciently strong assumptions to yield deﬁnitive ﬁndings on deter-

rence. For example, a common assumption is that sanctions are random

across states or years, as they would be if sanctions had been randomly as-

signed in an experiment. Another common assumption is that the response

of criminality to sanctions is homogeneous across states and years. Some

studies use instrumental variables to identify deterrent effects, but this

requires yet other assumptions. The use of strong assumptions hides the

problem that the study of deterrence is plagued by model uncertainty and

that many of the assumptions used in the research lack credibility.

NEXT STEPS FOR RESEARCH

The earlier NRC committee concluded that it was “skeptical that the

death penalty [as practiced in the United States] can ever be subjected to the

kind of statistical analysis that would validly establish the presence or ab-

sence of a deterrent effect” (National Research Council, 1978, p. 62). The

present committee is not so pessimistic and offers several recommendations

for addressing the shortcomings in research to date on capital punishment.

They include

1. collection of the data required for a more complete speciﬁcation of

both the capital and noncapital components of the sanction regime

for murder;

2. research on how potential murderers perceive the sanction regime

for murder; and

3. use of methods that makes less strong and more credible assump-

tions to identify or bound the effect of capital punishment on

homicides.

Deterrence and the Death Penalty

8 DETERRENCE AND THE DEATH PENALTY

In addition, the committee suggests research on how the presence of capital

punishment in a sanctions regime affects the administration of the regime

and how the homicide rate affects the statutory deﬁnition of the sanction

regime and its administration.

The committee does not expect that advances in new data on sanction

regimes and obtaining knowledge of sanctions risk perceptions will come

quickly or easily. However, data collection on the noncapital component of

the sanction regime need not be entirely complete to be useful. Moreover,

even if research on perceptions of the risk of capital punishment cannot

resolve all major issues, some progress would be an important step forward.

The ultimate success of the research may depend on the speciﬁc ques-

tion that is addressed. Questions of interest include

• if or how the legal status of the death penalty affects homicide

rates,

• if or how the intensity of use of the death penalty affects homicide

rates, and

• iforhowexecutionsaffecthomicideratesintheshortrun.

Some but not all of these questions may be informed by successful applica-

tion of the committee’s suggested lines of research.

Although evaluation of research on the deterrent effect of noncapital

sanctions was not part of the committee’s charge, we note that the methods

and approaches used to study capital and noncapital sanction effects on

crime overlap. We were charged with making suggestions for advancing

research on the latter issue. Thus, the research and data collection sugges-

tions above are framed in the broader context of research on the effect on

crime rates of both capital and noncapital sanctions.

We think this aspect of our charge is particularly important. Although

capital punishment is a highly contentious public policy issue, policies on

prison sanctions and their enforcement are the most important components

of the nation’s response to crime. Thus, even if the research agenda we

outline is not ultimately successful in illuminating some aspects of the ef-

fect of capital punishment on homicide, advancing knowledge on the crime

prevention effects of noncapital sanctions and their enforcement can make

major contributions to important policy issues.

REFERENCE

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.Washing-

ton, DC: National Academy Press.

Deterrence and the Death Penalty

Introduction

n 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.

THE CURRENT DEBATE

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

Deterrence and the Death Penalty

10 DETERRENCE AND THE DEATH PENALTY

homicide rates and the legal status or the actual implementation of the

death penalty.

The studies have reached widely varying, even contradictory, conclu-

sions, 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 mar-

gin 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 exis-

tence 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-

Deterrence and the Death Penalty

INTRODUCTION 11

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 obser-

vational 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).

COMMITTEE CHARGE AND SCOPE OF WORK

The Committee on Deterrence and the Death Penalty was organized

against this backdrop of conﬂicting claims about the effect of capital pun-

ishment on homicide rates, with the following charge:

Deterrence and the Death Penalty

12 DETERRENCE AND THE DEATH PENALTY

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 ac-

tual practice of the death penalty on criminal homicide rates. The commit-

tee 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: