|
||||||||||||||||
|
||||||||||||||||
|
|
|||||||||||||||
|
| ||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 1
Summary
I
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:
1
OCR for page 2
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 specified amount
or has no effect on the homicide rate should not influence 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
OCR for page 3
3
SUMMARY
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
OCR for page 4
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.
Specification 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-
OCR for page 5
5
SUMMARY
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.
OCR for page 6
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 Unverifiable 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
OCR for page 7
7
SUMMARY
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.
OCR for page 8
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.
