Expert panel concludes that death penalty deterrence studies are inconclusive

As reported in this Los Angeles Times article, headlined "Death penalty a deterrent to murder? Study says evidence unclear," a special committee formed by the National Research Council yesterday released a report which concluded that extant studies about the death penalty and deterrence are inconclusive. Here are the basics:

The Committee of Deterrence and the Death Penalty concluded that studies on the death penalty and its potential effect on homicide rates — both pro and con — contain fundamental flaws that essentially make them moot.  For example, the studies do not include the effects of other forms of punishment — such as life in prison without possibility of parole, and whether it too acts as a deterrent.  The studies, study authors wrote, don’t “consider how the capital and noncapital components of a regime combine in affecting the behavior of potential murderers.”

In other words, previous studies don’t determine whether potential killers think about the possibility of spending their lives in prison or ending up on death row before they commit their crimes.  The lack of comprehensive information makes the research inconclusive, the study authors said.  “We recognize this conclusion will be controversial to some, but nobody is well served by unfounded claims about the death penalty,” committee Chairman Daniel Nagin said in a telephone news conference.  “Nothing is known about how potential murderers actually perceive their risk of punishment,” he said.

Nagin, a professor of public policy and statistics at Carnegie Mellon University, said more data were needed on the full range of penalties across the country before they are cited as basis for changing public policy.  More precise data collection is needed, Nagin said, because the issue is so fundamentally difficult to study.  For example, it’s scientifically impossible to know exactly what was going on in someone’s head when they killed someone — even if they are interviewed about it afterward.

The study also concludes that data alone can’t reveal what the homicide rate in a state with the death penalty would be if it didn’t have the punishment — and vice versa.

This official press release about the report and its findings provides an additional summary; these starting paragraphs from the press release also provide a link to the full report:

Research to date on the effect of capital punishment on homicide rates is not useful in determining whether the death penalty increases, decreases, or has no effect on these rates, says a new report from the National Research Council.  The committee that wrote the report evaluated studies conducted since a four-year moratorium on the death penalty was lifted in 1976, and it found that the studies do not provide evidence for or against the proposition that the death penalty affects homicide rates.  These studies should not be used to inform judgments about the effect of the death penalty on homicide, and should not serve as a basis for policy decisions about capital punishment, the committee said.

The lack of evidence about the deterrent effect of capital punishment — whether it is positive, negative, or zero — should not be construed as favoring one argument over another, the report stresses.  "Fundamental flaws in the research we reviewed make it of no use in answering the question of whether the death penalty affects homicide rates," said Daniel S. Nagin, Teresa and H. John Heinz III University Professor of Public Policy and Statistics at Carnegie Mellon University Pittsburgh, and chair of the committee that wrote the report.  "We recognize that this conclusion may be controversial to some, but no one is well-served by unsupportable claims about the effect of the death penalty, regardless of whether the claim is that the death penalty deters homicides, has no effect on homicide rates or actually increases homicides."

The key question, the report says, is whether capital punishment is less or more effective as a deterrent than alternative punishments, such as a life sentence without the possibility of parole.  Yet none of the research that has been done accounted for the possible effect of noncapital punishments on homicide rates.