Expert Commentary

The research on capital punishment: Recent scholarship and unresolved questions

2014 review of research on capital punishment, including studies that attempt to quantify rates of innocence and the potential deterrence effect on crime.

Over the past year the death penalty has again come into focus as a major public policy and political issue, catalyzed by several high-profile events.

The botched execution of convicted murderer and rapist Clayton Lockett in Oklahoma in 2014 was seen as a potential turning point in the debate, bringing increased attention to the mechanisms by which persons are executed. That was followed by a number of other closely scrutinized cases, and the year ended with few executions relative to years past. On December 31, 2014, Maryland Gov. Martin O’Malley commuted the sentences of the remaining four prisoners on death row in that state. In 2013, Maryland became the 18th state to abolish the death penalty after Connecticut in 2012 and New Mexico in 2009.

Meanwhile, polling data suggests some softening of public attitudes, though the majority Americans continue to support capital punishment. Gallop noted in October 2014 that the level of public support (60%) is at its lowest in 40 years. A Washington Post-ABC News poll in mid-2014 found that more Americans support life sentences, rather than the death penalty, for convicted murderers. Further, recent polls from the Pew Research Center indicate that only a bare majority of Americans now support capital punishment, 55%, down from 78% in 1996.

Scholarly research sheds light on a number of important aspects of this issue:

False convictions

One 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.”

Other factors to consider

The question of what kinds of “mitigating” factors should prevent the criminal justice system from moving forward with an execution remains hotly disputed. A 2014 paper published in the Hastings Law Journal, “The Failure of Mitigation?” by scholars at the University of North Carolina and DePaul University, investigates recent executions of persons with possible mental or intellectual disabilities. The authors reviewed 100 cases and conclude that the “overwhelming majority of executed offenders suffered from intellectual impairments, were barely into adulthood, wrestled with severe mental illness, or endured profound childhood trauma.”

Two significant recommendations for reforming the existing process also are supported by some academic research. A 2010 study by Pepperdine University School of Law published in Temple Law Review, “Unpredictable Doom and Lethal Injustice: An Argument for Greater Transparency in Death Penalty Decisions,” surveyed the decision-making process among various state prosecutors. At the request of a state commission, the authors first surveyed California district attorneys; they also examined data from the other 36 states that have the death penalty. The authors found that prosecutors’ capital punishment filing decisions remain marked by local “idiosyncrasies,” meaning that “the very types of unfairness that the Supreme Court sought to eliminate” beginning in 1972 may still “infect capital cases.” They encourage “requiring prosecutors to adhere to an established set of guidelines.” Finally, there has been growing support for taping interrogations of suspects in capital cases, so as to guard against the phenomenon of false confessions.

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, death penalty, capital punishment