20 April 2012 :

A report ("Deterrence and the Death Penalty") released by the prestigious National Research Council of the National Academies 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." 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.