other sources (but see discussion below), it opens other possible interpretations of the result. Even if a short-term effect could be established, it would be difficult to determine whether homicides were actually prevented or simply displaced in time. This possibility creates a fundamental conundrum: the study of short time frames increases the plausibility of the displacement in time interpretation, and the study of longer time frames increases the risk of confounding by other factors.

It is vital to understand that event studies do not speak to the question of whether and how a state’s sanction regime affects its homicide rate. The simplest illustration of this point involves the interpretation of a study that fails to find evidence that an execution event affects the homicide rate. Consider, for example, a study of the first execution after an extended moratorium. Suppose that the study convincingly demonstrated that the execution was not followed by any change in the homicide rate. One interpretation of this result is that capital punishment has no deterrent effect. However, another possibility is that the deterrent effect is large but that it was anticipated in advance of the execution due to the publicity given to the upcoming event. Both possibilities are logical and plausible, but they are not distinguishable by the event study methodology.

Alternatively, suppose that an event study found that homicides are reduced in the immediate aftermath of an execution and not just displaced in time. To generalize from this single execution requires consideration of the context in which the execution occurred. If it was the first execution after an extended moratorium, it is problematic to assume that such an effect would recur for subsequent executions. More generally, the effect of any given execution may depend on the proximity in time of that execution to other executions and to the frequency of executions more generally. For example, if an execution event study established convincingly that it averted one homicide that week, it does not follow that each additional execution would avert one more homicide. To complicate matters further, the effect of any one execution may depend on the identity of the person executed (e.g., an infamous serial killer or a person for whom there is some public sympathy) and the amount of publicity given to the execution.

The problem of generalizing from the findings of even a convincing event study is indicative of still another fundamental committee concern with all the time-series studies. The researchers who carry out such studies never clearly specify why potential murderers respond to execution events. Do potential murderers respond to the shock value of execution? If so, would the magnitude of the shock value change with each additional execution? One possibility is that the shock value might increase, perhaps because of reinforcement. Alternatively, it might decrease, perhaps because a potential murderer becomes inured to executions. Still another possibility is



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement