lead to very different inferences on the effects of capital punishment, and none of them is inherently any more justifiable than any other.
Additional data and research on sanction regimes and risk perceptions may serve to reduce this form of model uncertainty. However, even if these uncertainties are fully reconciled, a more fundamental problem is that the outcomes of counterfactual sanction policies are unobservable. That is, there is no way to determine what would have occurred if a given state had a different sanction regime. In light of this observational problem, the available data cannot reveal the effect of capital punishment itself since the policy-relevant question is whether capital punishment deters homicides relative to other sanction regimes. That is, the data alone cannot reveal what the homicide rate in a state without (with) a capital punishment regime would have been had the state (not) had such a regime.
The standard procedure in capital punishment research has been to impose sufficiently strong assumptions to yield definitive findings on deterrence. For example, a common assumption is that sanctions are random across states or years, as they would be if sanctions had been randomly assigned in an experiment. Another common assumption is that the response of criminality to sanctions is homogeneous across states and years. Some studies use instrumental variables to identify deterrent effects, but this requires yet other assumptions. The use of strong assumptions hides the problem that the study of deterrence is plagued by model uncertainty and that many of the assumptions used in the research lack credibility.
The earlier NRC committee concluded that it was “skeptical that the death penalty [as practiced in the United States] can ever be subjected to the kind of statistical analysis that would validly establish the presence or absence of a deterrent effect” (National Research Council, 1978, p. 62). The present committee is not so pessimistic and offers several recommendations for addressing the shortcomings in research to date on capital punishment. They include
1. collection of the data required for a more complete specification of both the capital and noncapital components of the sanction regime for murder;
2. research on how potential murderers perceive the sanction regime for murder; and
3. use of methods that makes less strong and more credible assumptions to identify or bound the effect of capital punishment on homicides.