that the null hypothesis is correct. For the evidence of even a small effect to be credible, it requires a demonstration, first and foremost, that the effect is based on a sound research design. Estimates that lack credibility are not informative regardless of the consistency of their estimated size. The amount of the effect must also be small in size and estimated with good precision, for example, by being contained within a tight confidence interval.
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.
We stress, however, as noted above, that 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 societal debate about deterrence and the death penalty.
In this chapter, we elaborate on these deficiencies that form the basis for this conclusion and cautiously offer some ideas on potential remedies. With regard to remedies, our report provides a somewhat less pessimistic perspective than did the earlier National Research Council (1978, p. 63) report: “[T]he Panel considers that research on this topic is not likely to produce findings that will or should have much influence on policymakers.”
The committee does not expect that advances in collecting data on sanction regimes and obtaining knowledge of sanctions risk perceptions will come quickly or easily. However, data collection on the noncapital component of the sanction regime need not be entirely complete to be useful. And even if research on perceptions of the risk of capital punishment cannot resolve all major issues, some progress would be an important step forward. Even if these advances prove unsuccessful in providing useful information on the incremental deterrent effect of capital punishment in relation to a lengthy prison sentence, the committee believes that there are potentially major benefits from new data collection, theory, and methodology for study of the effect of noncapital sanctions on crimes not subject to the death penalty. As discussed in Chapter 1, because of the overlap in the methods and data used in studies of capital punishment and in broader studies on the effects of sanctions on crime, our charge included a provision for recommending research that might advance that broader research literature, and we do so in the rest of this chapter.