understood, the relevant question regarding the deterrent effect of capital punishment is the differential or marginal deterrent effect of execution over the deterrent effect of other available or commonly used penalties. We emphasize “differential” because it is important to recognize that the alternative to capital punishment is not no punishment or a minor punishment such as probation. Instead, it is a lengthy prison sentence—often life without the possibility of parole.
The theory of deterrence is predicated on the idea that if state-imposed sanction costs are sufficiently severe, certain, and swift, criminal activity will be discouraged. Concerning the severity dimension, a necessary condition for state-sanctioned executions to deter crime is that, at least for some, capital punishment is deemed an even worse fate than the possibility of a lifetime of imprisonment.1 Severity alone, however, cannot deter. There must also be some possibility that the sanction will be incurred if the crime is committed. For that to happen, the offender must be apprehended, charged, successfully prosecuted, and sentenced by the judiciary. As discussed in Chapter 2, none of these successive stages in processing through the criminal justice system is certain. Thus, another key concept in deterrence theory is the certainty of punishment. Many of the studies of the deterrent effect of capital punishment attempt to estimate whether homicide rates seem to be affected by variation in various measures of the likelihood of execution beyond the likelihood of apprehension and conviction.
Across the social science disciplines, the concepts of certainty and severity have been made operational in deterrence research in very different ways. In Becker’s (1968) seminal economic formulation of criminal decision making, individual perceptions of certainty and severity are assumed to correspond to reality. The decision to commit a crime is also assumed to correspond with a precisely formulated set of axioms that define rational decision making. In contrast, among criminologists, models of criminal decision making are less mathematically formalized and place great emphasis on the role of perceptions. These models also explicitly acknowledge that perceptions of certainty and severity may diverge substantially from reality and are probably heavily influenced by experience with the criminal justice system (Cook, 1980; Nagin, 1998). More recent theorizing about criminal decision making also incorporates insights from behavioral economics on biases in risk perceptions to better model the linkage between sanction risk perceptions and reality (Durlauf and Nagin, 2011; Kleiman, 2009; Pogarsky, 2009). For example, prospect
1Another way sanctions may prevent crime is by making it physically impossible for the offender to commit another crime. Execution achieves this end by the death of the offender. Note, however, that a death sentence will not, on the margin, be more effective in preventing crime (outside a prison) than the incapacitation that accompanies a sentence of life imprisonment without parole.