Criminal Justice

The research on capital punishment: Recent scholarship and unresolved questions

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Last updated: May 1, 2014

San Quentin State Prison (Wikimedia)
San Quentin State Prison (Wikimedia)

News media outlets and commentators have been quick to label the botched execution of convicted murderer and rapist Clayton Lockett in Oklahoma a potential turning point in the debate over the death penalty. But the debate over capital punishment — the justice of it, in addition to questions about its mechanics — has long simmered in America. A May 2014 Gallup poll conducted after the Oklahoma situation showed that the views of Americans are virtually unchanged.

In 2013, Maryland became the 18th state to abolish the death penalty after Connecticut in 2012 and New Mexico in 2009. In fact, recent polls from the Pew Research Center indicate that fewer and fewer Americans support the practice, with those in favor of capital punishment falling from 62% in 2011 to 55% in 2013. Gallup polls also record a modest downward trend in support over the past four decades. Still, the death penalty remains a controversial policy issue for a variety of reasons, including personal ethical beliefs, cost considerations and the potential for bias in legal decisions — not only by juries, but by prosecutors implementing inconsistent rules to bring such cases to trial.

False convictions

Another key reason for the contentious debate is the concern that states are executing innocent people. How many people are unjustly facing the death penalty? By definition, it is difficult to obtain a reliable answer to this question. Presumably if judges, juries, and law enforcement were always able to conclusively determine who was innocent, those defendants would simply not be convicted in the first place. When capital punishment is the sentence, however, this issue takes on new importance.

Some believe that when it comes to death-penalty cases, this is not a huge cause for concern. In his concurrent opinion in the 2006 Supreme Court case Kansas v. Marsh, Justice Antonin Scalia suggested that the execution error rate was minimal, around 0.027%. However, a 2014 study in the Proceedings of the National Academy of Sciences suggests that the figure could be higher. Authors Samuel Gross (University of Michigan Law School), Barbara O’Brien (Michigan State University College of Law), Chen Hu (American College of Radiology) and Edward H. Kennedy (University of Pennsylvania School of Medicine) examine data from the Bureau of Justice Statistics and the Department of Justice relating to exonerations from 1973 to 2004 in an attempt to estimate the rate of false convictions among death row defendants. (Determining innocence with full certainty is an obvious challenge, so as a proxy they use exoneration — “an official determination that a convicted defendant is no longer legally culpable for the crime.”) In short, the researchers ask: If all death row prisoners were to remain under this sentence indefinitely, how many of them would have eventually been found innocent (exonerated)?

Death penalty attitudes (Pew)To answer this question, they use a statistical method called “survival analysis,” a technique often used to calculate the effectiveness of medical treatments. After examining 7,482 cases, they estimate that 1 in 25 death row inmates are wrongly convicted. They conclude: “With an error rate at trial over 4%, it is all but certain that several of the 1,320 defendants executed since 1977 were innocent.” The authors explain that translating this conviction error rate into a reliable estimate of the number of innocents who have been executed is difficult, but “[they] do know that the rate of error among death sentences is far greater than Justice Scalia’s reassuring 0.027%.” However, they still believe it to be a low number: “Our data and the experience of practitioners in the field both indicate that the criminal justice system goes to far greater lengths to avoid executing innocent defendants than to prevent them from remaining in prison indefinitely.”

Interestingly, the authors also note that advances in DNA identification technology are unlikely to have a large impact on false conviction rates because DNA evidence is most often used in cases of rape rather than homicide. To date, only about 13% of death row exonerations were the result of DNA testing. The Innocence Project, a litigation and public policy organization founded in 1992, has been deeply involved in many such cases.

Death penalty deterrence effects: What do we know?

A chief way proponents of capital punishment defend the practice is the idea that the death penalty deters other people from committing future crimes. For example, research conducted by John J. Donohue III (Yale Law School) and Justin Wolfers (University of Pennsylvania) applies economic theory to the issue: If people act as rational maximizers of their profits or well-being, perhaps there is reason to believe that the most severe of punishments would serve as a deterrent. (The findings of their 2009 study on this issue, “Estimating the Impact of the Death Penalty on Murder,” are inconclusive.) In contrast, one could also imagine a scenario in which capital punishment leads to an increased homicide rate because of a broader perception that the state devalues human life. It could also be possible that the death penalty has no effect at all because information about executions is not diffused in a way that influences future behavior.

In 1978 — two years after the Supreme Court issued its decision reversing a previous ban on the death penalty (Gregg v. Georgia) — the National Research Council (NRC) published a comprehensive review of the current research on capital punishment to determine whether one of these hypotheses was more empirically supported than the others. The NRC concluded that “available studies provide no useful evidence on the deterrent effect of capital punishment.”

Researchers have subsequently used a number of methods in an effort to get closer to an accurate estimate of the deterrence effect of the death penalty. Many of the studies have reached conflicting conclusions, however. To conduct an updated review, the NRC formed the Committee on Deterrence and the Death Penalty, comprised of academics from economics departments and public policy schools from institutions around the country, including the Carnegie Mellon University, University of Chicago and Duke University.

In 2012, the Committee published an updated report that concluded that not much had changed in recent decades: “Research conducted in the 30 years since the earlier NRC report has not sufficiently advanced knowledge to allow a conclusion, however qualified, about the effect of the death penalty on homicide rates.” The report goes on to recommend that none of the reviewed reports be used to influence public policy decisions on the death penalty.

Why has the research not been able to provide any definitive answers about the impact of the death penalty? One general challenge is that when it comes to capital punishment, a counter-factual policy is simply not observable. You cannot simultaneously execute and not execute defendants, making it difficult to isolate the impact of the death penalty. The Committee also highlights a number of key flaws in the research designs:

  • There are both capital and non-capital punishment options for people charged with serious crimes. So, the relevant question on the deterrent effect of capital punishment specifically “is the differential deterrent effect of execution in comparison with the deterrent effect of other available or commonly used penalties.” None of the studies reviewed by the Committee took into account these severe, but noncapital punishments, which could also have an effect on future behaviors and could confound the estimated deterrence effect of capital punishment.
  • “They use incomplete or implausible models of potential murderers’ perceptions of and response to the capital punishment component of a sanction regime”
  • “The existing studies use strong and unverifiable assumptions to identify the effects of capital punishment on homicides.”

In a 2012 study, “Deterrence and the Dealth Penalty: Partial Identificaiton Analysis Using Repeated Cross Sections,” authors Charles F. Manski (Northwestern University) and John V. Pepper (University of Virginia) focus on the third challenge. They note: “Data alone cannot reveal what the homicide rate in a state without (with) a death penalty would have been had the state (not) adopted a death penalty statute. Here, as always when analyzing treatment response, data must be combined with assumptions to enable inference on counterfactual outcomes.”

Number of persons executed in the U.S., 1930-2011 (BJS)The authors examine state-level data from 1975 and 1977. (In 1975, the death penalty became illegal across the United States, in 1977 the ban had been overturned and 32 states had legal death penalty statutes.) This allows the researchers to compare different homicide rates for each state and each year. By conducting this analysis using a range of assumptions, they find that the deterrence effect of the death penalty strongly depends on those particular assumptions. They conclude: “Imposing certain assumptions implies that adoption of a death penalty statute increases homicide, but other assumptions imply that the death penalty deters it. Thus, society at large can draw strong conclusions only if there is a consensus favoring particular assumptions.”

However, even though the authors do not arrive at a definitive conclusion, the National Research Council Committee notes that this type of research holds some value: “Rather than imposing the strong but unsupported assumptions required to identify the effect of capital punishment on homicides in a single model or an ad hoc set of similar models, approaches that explicitly account for model uncertainty may provide a constructive way for research to provide credible albeit incomplete answers.”

Another strategy researchers have taken is to limit the focus of studies on potential short-term effects of the death penalty. In a 2009 paper, “The Short-Term Effects of Executions on Homicides: Deterrence, Displacement, or Both?” authors Kenneth C. Land and Hui Zheng of Duke University, along with Raymond Teske Jr. of Sam Houston State University, examine monthly execution data (1980-2005) from Texas, “a state that has used the death penalty with sufficient frequency to make possible relatively stable estimates of the homicide response to executions.” They conclude that “evidence exists of modest, short-term reductions in the numbers of homicides in Texas in the months of or after executions.” Depending on which model they use, these deterrent effects range from 1.6 to 2.5 homicides.

The NRC’s Committee on Deterrence and the Death Penalty commented on the findings, explaining: “Land, Teske and Zheng (2009) should be commended for distinguishing between periods in Texas when the use of capital punishment appears to have been erratic and when it appears to have been systematic. But they fail to integrate this distinction into a coherently delineated behavioral model that incorporates sanctions regimes, salience, and deterrence. And, as explained above, their claims of evidence of deterrence in the systematic regime are flawed.”

A more recent paper (2012) from the three authors, “The Differential Short-Term Impacts of Executions on Felony and Non-Felony Homicides,” addresses some of these concerns. Published in Criminology and Public Policy, the paper reviews and updates some of their earlier findings by exploring “what information can be gained by disaggregating the homicide data into those homicides committed in the course of another felony crime, which are subject to capital punishment, and those committed otherwise.” The results produce a number of different findings and models, including that “the short-lived deterrence effect of executions is concentrated among non-felony-type homicides.”

Related reading: For an international perspective on capital punishment, see Amnesty International’s 2013 report; for more information on the evolution of U.S. public opinion on the death penalty, see historical trends from Gallup.


Keywords: crime, prisons

Writer: | May 1, 2014

Analysis assignments

Read the issue-related New Yorker article titled "Trial by Fire: Did Texas Execute an Innocent Man?"

"Trial by Fire: Did Texas Execute an Innocent man?" - See more at:
  1. What key insights from the news article  in this lesson should reporters be aware of as they cover these issues?

Read the study-related Associated Press article titled "Study: 1 in 25 Death Cases Likely Innocent."

  1. Reporter's use of the study: Evaluate what the reporter chose to include and exclude from the study. Would the audience have acquired a clear understanding of the study's findings and limits from this article?
  2. Reporter's use of other material: Assess the material in the article that is not derived from the study. For example: Does the reporter place the study in the context of other research and to what effect? Does the reporter include reactions to the study from other researchers or interested parties (e.g., political groups, business leaders, or community members) and are their credentials or possible biases made clear?

Read the full study titled “Deterrence and the Dealth Penalty: Partial Identificaiton Analysis Using Repeated Cross Sections.”

  1. What are the study's key technical terms? Which ones need to be put into language a lay audience can understand?
  2. Do the study’s authors put the research into context and show how they are advancing the state of knowledge about the subject? If so, what did the previous research indicate?
  3. What is the study’s research method? If there are statistical results, how did the scholars arrive at them?
  4. Evaluate the study's limitations. (For example, are there weaknesses in the study's data or research design?)
  5. How could the findings be misreported or misinterpreted by a reporter? In other words, what are the difficulties in conveying the data accurately? Give an example of a faulty headline or story lead.

Newswriting and digital reporting assignments

  1. Write a lead, headline or nut graph based on the study.
  2. Spend 60 minutes exploring the issue by accessing sources of information other than the study. Write a lead (or headline or nut graph) based on the study but informed by the new information. Does the new information significantly change what one would write based on the study alone?
  3. Compose two Twitter messages of 140 characters or fewer accurately conveying the study’s findings to a general audience. Make sure to use appropriate hashtags.
  4. Choose several key quotations from the study and show how they would be set up and used in a brief blog post.
  5. Map out the structure for a 60-second video segment about the study. What combination of study findings and visual aids could be used?
  6. Find pictures and graphics that might run with a story about the study. If appropriate, also find two related videos to embed in an online posting. Be sure to evaluate the credibility and appropriateness of any materials you would aggregate and repurpose.

Class discussion questions

  1. What is the study’s most important finding?
  2. Would members of the public intuitively understand the study’s findings? If not, what would be the most effective way to relate them?
  3. What kinds of knowledgeable sources you would interview to report the study in context?
  4. How could the study be “localized” and shown to have community implications?
  5. How might the study be explained through the stories of representative individuals? What kinds of people might a reporter feature to make such a story about the study come alive?
  6. What sorts of stories might be generated out of secondary information or ideas discussed in the study?


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