Studies on Deterrence, Debunked

The Criminal Justice Legal Foundation has collected many of the recent, controversial deterrence studies, including ones by Hashem Dezhbakhsh, Paul H. Rubin, Joanna M. Shepherd, H. Naci Mocan & R. Kaj Gittings and others claiming a deterrent effect to the death penalty. These studies may be found here.

National Research Council, Deterrence and the Death Penalty

On April 18, 2012, the prestigious National Research Council of the National Academies released “Deterrence and the Death Penalty,” a report based on a review of more than three decades of research concluded that studies claiming a deterrent effect on murder rates from the death penalty are fundamentally flawed. The report concluded:

The committee concludes that research to date on the effect of capital punishment on homicide 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 punishment decreases or increases the homicide rate by a specified amount or has no effect on the homicide rate should not influence policy judgments about capital punishment. (emphasis added).

Criminologist Daniel Nagin of Carnegie Mellon, who chaired the panel of experts, said, “We recognize this conclusion will be controversial to some, but nobody is well served by unfounded claims about the death penalty. Nothing is known about how potential murderers actually perceive their risk of punishment.”

The report found three fundamental flaws with existing studies on deterrence:

• The studies do not factor in the effects of noncapital punishments that may also be imposed.
• The studies use incomplete or implausible models of potential murderers’ perceptions of and response to the use of capital punishment.
• Estimates of the effect of capital punishment are based on statistical models that make assumptions that are not credible.

The National Resource Council’s conclusions are supported by a number of earlier studies.

Death and Deterrence Redux: Science, Law and Causal Reasoning on Capital Punishment

In an article in the Ohio State Journal of Criminal Law, Dr. Jeffrey Fagan of Columbia University describes numerous serious errors in recent deterrence studies, including improper statistical analyses and missing data and variables that are necessary to give a full picture of the criminal justice system. Fagan writes, “There is no reliable, scientifically sound evidence that [shows that executions] can exert a deterrent effect…. These flaws and omissions in a body of scientific evidence render it unreliable as a basis for law or policy that generate life-and-death decisions. To accept it uncritically invites errors that have the most severe human costs.” Since the landmark Supreme Court decision in Furman v. Georgia in 1972, dozens of studies have been performed to determine whether future murderers are deterred by the death penalty. In the past five years, Fagan writes, a “new wave” of studies has emerged, claiming that each execution prevents 3-32 murders, depending on the study. Some of these studies tie pardons, commutations, exonerations, and even irrational murders of passion to increases in murder rates. While many of these studies have appeared in academic journals, they have been given an uncritical and favorable reception in leading newspapers. Fagan takes issue with this lack of serious and adequate peer review by fellow researchers. He analyzed this research and found that “this work fails the tests of rigorous replication and robustness analysis that are the hallmarks of good science.”(4 Ohio State Journal of Criminal Law 255 (2006))

The Death Penalty: No Evidence for Deterrence

In an article entitled The Death Penalty: No Evidence for Deterrence, John Donnohue and Justin Wolfers examined statistical studies that claimed to show a deterrent effect from the death penalty. The authors conclude that the estimates claiming that the death penalty saves numerous lives “are simply not credible.” In fact, the authors state that using the same data and proper methodology could lead to the exact opposite conclusion: that is, that the death penalty actually increases the number of murders. The authors state: “We show that with the most minor tweaking of the [research] instruments, one can get estimates ranging from 429 lives saved per execution to 86 lives lost. These numbers are outside the bounds of credibility.” (The Economists’ Voice, April 2006).

The Uses and Abuses of Empirical Evidence in the Death Penalty Debate

In 2005, the Stanford Law Review published an article entitled Uses and Abuses of Empirical Evidence in the Death Penalty Debate. The article examines and performs comparison tests on studies that have claimed a deterrent effect to the death penalty. Authors John J. Donohue of Yale Law School and Justin Wolfers of the University of Pennsylvania state their goal and conclusions: “Aggregating over all of our estimates, it is entirely unclear even whether the preponderance of evidence suggests that the death penalty causes more or less murder.” (58 Stanford Law Review 791 (2005)).

The Death Penalty Meets Social Science: Deterrence and Jury Behavior Under New Scrutiny

Robert Weisberg, a professor at Stanford University’s School of Law, examined studies on deterrence and the death penalty, as well as other social science research regarding capital punishment in the U.S. In The Death Penalty Meets Social Science: Deterrence and Jury Behavior Under New Scrutiny, Weisberg notes that many of the new studies claiming to find that the death penalty deters murder have been legitimately criticized for omitting key variables and for not addressing the potential distorting effect of one high-executing state, Texas. Later in the article, Weisberg examines studies on race-of-victim discrimination and on capital jurors. This article will appear in the forthcoming edition of the Annual Review of Law and Social Science. (1 Annual Review of Law and Social Science 151 (2005)).

Public Policy Choices on Deterrence and the Death Penalty: A Critical Review of New Evidence

In testimony before the Massachusetts Joint Committee on the Judiciary regarding proposed legislation to initiate a “foolproof” death penalty, Columbia Law School Professor Jeffrey Fagan analyzed studies that claimed that capital punishment deters murders. He stated that the studies “fall apart under close scrutiny.” Fagan noted that the studies are fraught with technical and conceptual errors, including inappropriate methods of statistical analysis, failures to consider all relevant factors that drive murder rates, missing data on key variables in key states, weak to non-existent tests of concurrent effects of incarceration, and other deficiencies. “A close reading of the new deterrence studies shows quite clearly that they fail to touch this scientific bar, let alone cross it,” Fagan said as he told members of the committee that the recent deterrence studies fell well short of the demanding standards of social science research. (J. Fagan, Public Policy Choices on Deterrence and the Death Penalty: A Critical Review of New Evidence, testimony before the Joint Committee on the Judiciary of the Massachusetts Legislature on House Bill 3934, July 14, 2005).

New Claims about Executions and General Deterrence: Deja Vu All Over Again?

A study conducted by Professor Richard Berk of the UCLA Department of Statistics identified significant statistical problems with the data analysis used to support studies claiming to show that executions deter crime in the United States. In “New Claims about Executions and General Deterrence: Deja Vu All Over Again?,” Professor Berk addresses the problem of “influence,” which occurs when a very small and atypical fraction of the available data dominates the statistical results of a study. He found that this statistical problem is found in a number of recent studies claiming to show that capital punishment deters violent crime. The UCLA study conducted by Berk found that in many instances the number of executions by state and year is the key explanatory variable used by researchers, despite the fact that many states in most years execute no one and few states in particular years execute more than five individuals. These values represent about 1% of the available observations that could have been used by researchers to draw conclusions for earlier studies claiming to find that capital punishment is a deterrent. In Professor Berk’s study, a re-analysis of the existing data shows that claims of deterrence are a statistical artifact of this anomalous 1%. (Published on UCLA’s Web site, July 19, 2004).