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The dilemma of whether to kill the killers comes up in only a small fraction of all U.S. homicides. The criteria for capital murder vary from state to state and even, inevitably, from case to case. In general, there must be "aggravating circumstances." These can be as specific as the murder of a fireman or one by an inmate serving a life sentence; as common as a homicide committed along with a lesser felony, like burglary; and as vague as Florida's law citing "especially heinous, atrocious or cruel" killings. It is estimated that about 10% of U.S. homicides currently qualify, or some 2,000 murders last year. Those killings are the ones the threat of capital punishment is meant to prevent.
The idea of deterrence can be quickly reduced to very personal rudiments: If I know I will be punished so severely, I will not commit the crime. The logic is undeniable. Yet in the thickets of real life and real crime, deterrence, while central to practically all punishment, is often very uncertain, and its effect on prospective murderers is especially unclear. Unfortunately, public discussion usually consists of flat-out pronouncements. Capital punishment, says Conservative Commentator William F. Buckley, "is a strong, plausible deterrent." No, declares New York Governor Cuomo, "there has never been any evidence that the death penalty deters." Neither is altogether wrong, but the stick-figure oversimplifications on both sides do a disservice to a complicated question.
The scholarly evidence is not quite as unequivocal as some abolitionists claim. But it does not make much of a case for deterrence. The most persuasive research compared the homicide rates of states that did and did not prescribe the death penalty. For instance, Michigan, which abolished capital punishment in 1847, was found to have had a homicide rate identical to adjacent states, Ohio and Indiana, that were executing. Similarly, Minnesota and Rhode Island, states with no death penalty, had proportionately as many killings as their respective neighbors, Iowa and Massachusetts, which had capital punishment. In 1939 South Dakota adopted and used the death penalty, and its homicide rate fell 20% over the next decade; North Dakota got along without capital punishment for the same ten years, and homicides dropped 40%.
Similar before-and-after studies in Canada, England and other countries likewise found nothing to suggest that capital punishment had deterred murderers any better than the prospect of long prison terms. And in Britain during the 1950s, a typical "lifer" actually served only about seven years, compared with a much tougher average U.S. "life" term today of 20 years. A comprehensive study in the U.S., by the National Academy of Sciences in 1978, also found that the death penalty had not proved its worth as a deterrent.
Were it not for the work of Economist Isaac Ehrlich, the deterrence debate would be entirely one-sided. Using econometric modeling techniques to build a "supply-and-demand" theory of murder, Ehrlich argued in a 1975 paper that capital punishment prevents more murders than do prison sentences. Because of the 3,411 executions carried out from