common characteristic of invoking strong, often unverifiable, assumptions in order to provide point estimates of the effect of capital punishment on homicides. A point estimate may offer the appearance of desirable certitude, but only at a high cost in credibility. Still another deficiency is inattention to potential feedbacks through which homicide rates, and crime rates more generally, may affect the specification and administration of a sanction regime while the regime simultaneously affects homicide rates. Recognition of potential feedbacks is relevant both to identify the direct effect of capital punishment on homicide rates and to predict the ultimate effect after feedbacks occur. Feedbacks affect the time-series and panel studies differently because of differences in the time frames of the data typically used in the two approaches—monthly, weekly, or even daily data in the time-series studies and annual data in the panel studies.

In light of these deficiencies, the committee has reached the following conclusion and recommendation:

CONCLUSION AND RECOMMENDATION: The committee concludes that research to date on the effect of capital punishment on homicide is not informative about whether capital punishment decreases, increases, or has no effect on homicide rates. Therefore, the committee recommends that these studies not be used to inform deliberations requiring judgments about the effect of the death penalty on homicide. Consequently, claims that research demonstrates that capital punishment decreases or increases the homicide rate by a specified amount or has no effect on the homicide rate should not influence policy judgments about capital punishment.

The committee was disappointed to reach the conclusion that research conducted in the 30 years since the National Research Council (1978) report on this subject has not sufficiently advanced knowledge to allow a conclusion, however qualified, about the effect of the death penalty on homicide rates. Yet this is our conclusion. Some studies play the useful role, either intentionally or not, of demonstrating the fragility of their claims to have found—or not to have found—deterrent effects. However, even these studies suffer from two intrinsic shortcomings that severely limit what can be learned from them about the effect of the death penalty on homicide rates from an examination of the death penalty as it has actually been administered in the United States in the past 35 years.

Commentary on research findings often pits studies claiming to find statistically significant deterrent effects against those finding no statistically significant effects, with the latter studies sometimes interpreted as implying that there is no deterrent effect. A fundamental point of logic about hypothesis testing is that failure to reject a null hypothesis does not imply



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