sanctions on crime more generally. Our negative conclusion on the informativeness of the evidence on the former issue should not be construed as extending to the latter issue because the committee did not review the very large body of evidence on the deterrent effect of noncapital sanctions.

SHORTCOMINGS IN EXISTING RESEARCH

The post-Gregg studies are usefully divided into two categories based on the type of data analyzed. One category, which we call panel data studies, analyzes sets of states or counties measured over time, usually from about 1970 to 2000. These studies relate homicide rates to variations over time and across states or counties in the legal status of capital punishment and/or the frequency of executions. The second category, which we call time-series studies, generally studies only a single geographic unit. The geographic unit may be as large as a nation or as small as a city. These studies usually examine whether there are short-term changes in homicide rates in that geographic unit in the aftermath of an execution.

As noted above, research on the effect of capital punishment on homicide suffers from two fundamental flaws that make them uninformative about the effect of capital punishment on homicide rates: they do not specify the noncapital sanction components of the sanction regime for the punishment of homicide, and they use incomplete or implausible models of potential murderers’ perceptions of and response to the capital punishment component of a sanction regime. In addition, the existing studies use strong and unverifiable assumptions to identify the effects of capital punishment on homicides.

Specification of the Sanction Regime for Homicide

The sanction regime for homicide comprises both the capital and noncapital sanctioning options that are available for its punishment and the policies governing the administration of these options. The relevant question regarding the deterrent effect of capital punishment is the differential deterrent effect of execution in comparison with the deterrent effect of other available or commonly used penalties. We emphasize “differential” because it is important to recognize that even in states that make the most intense use of capital punishment, most convicted murderers are not sentenced to death but to a lengthy prison sentence—often life without the possibility of parole.

None of the studies that we reviewed (both those using a panel approach and those using time-series approaches) accounted for the severity of noncapital sanctions in their analyses. As discussed in Chapters 4 and 6, there are sound reasons to expect that the severity of the noncapital sanc-



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