parole. One major deficiency in all the existing studies is that none specify the noncapital sanction components of the sanction regime for the punishment of homicide. Another major deficiency is the use of incomplete or implausible models of potential murderers’ perceptions of and response to the capital punishment component of a sanction regime. Without this basic information, it is impossible to draw credible findings about the effect of the death penalty on homicide.
Commentary on research findings often pits studies claiming to find statistically significant deterrent effects against those finding no statistically significant effects, with the latter studies sometimes interpreted as implying that there is no deterrent effect. A fundamental point of logic about hypothesis testing is that failure to reject a null hypothesis does not imply that the null hypothesis is correct.
Our mandate was not to assess whether competing hypotheses about the existence of marginal deterrence from capital punishment are plausible, but simply to assess whether the empirical studies that we have reviewed provide scientifically valid evidence. In its deliberations and in this report, the committee has made a concerted effort not to approach this question with a prior assumption about deterrence. Having reviewed the research that purports to provide useful evidence for or against the hypothesis that the death penalty affects homicide rates, we conclude that it does not provide such evidence.
A lack of evidence is not evidence for or against the hypothesis. Hence, the committee does not construe its conclusion that the existing studies are uninformative as favoring one side or the other side in the long-standing debate about deterrence and the death penalty. The committee also emphasizes that deterrence is but one of many considerations relevant to rendering a judgment on whether the death penalty is good public policy.
Even though the scholarly evidence on the deterrent effect of capital punishment is too weak to guide decisions, this does not mean that people should have no views on capital punishment. Judgment about whether there is a deterrent effect is still relevant to policy, but that judgment should not be justified based on evidence from existing research on capital punishment’s effect on homicide. Just as important, the committee did not investigate the moral arguments for or against capital punishment or the empirical evidence on whether capital punishment is administered in a nondiscriminatory and consistent fashion. Nor did it investigate whether the risk of mistaken execution is acceptably small or how the cost of administering the death penalty compares to other sanction alternatives. All of these issues are relevant to making a judgment about whether the death penalty is good public policy.
Our charge was also limited to assessing the evidence on the deterrent effect of the death penalty on murder, not the deterrent effect of noncapital