Until 1975 researchers analyzing murder or criminal homicide statistics were unanimous in their conclusion: they found no empirical evidence to support the presumed unique deterrent effect of the death penalty and could discern no visible effect this penalty has on the rates of homicide. But in 1975 a

study reporting an opposite conclusion was published.¹⁴ The study was done by I. Ehrlich, an economist at the University of Chicago. Ehrlich used a set of assumptions to construct an econometric model, employed aggregate data and claimed to have found evidence that a capital execution would indeed deter some potential killers and perhaps save as many as eight lives:

"Empirical analysis suggests that on average the trade off between the execution of an offender and the lives of potential victims it might have saved was of the order of magnitude of one for eight for the period 1933-1967 in the U.S.A."

Not only was Ehrlich's claim at odds with the findings of all studies done in the U.S. and elsewhere but it was the first time ever a researcher has claimed to have been able to estimate the number of murders prevented by each execution. Ehrlich's findings received a great deal of publicity and were circulated widely by many police forces in the U.S.A. and Canada. More important still was their presentation as evidence in support of the death penalty before U.S. Courts. Until then the deterrence debate was largely an academic one and was rarely used in the courtroom. But in 1976 statistical evidence in support of the deterrence hypothesis was submitted to the U.S. Supreme Court in Fowler vs. North Carolina (428 U.S. 904, 1976 ). It was in this case that the Solicitor General submitted to the Court an amicus curiae brief citing Ehrlich's conclusion that capital punishment deters murder. And in another case: Gregg vs. Georgia (428 U.S. 153, 169, 1976 ) the Supreme Court ruled that "the punishment of death does not invariably violate the constitution" and added that for many murderers "the death penalty undoubtedly is a significant deterrent". One would have expected the Court to substantiate such claim with empirical evidence but the Court did not although such evidence was not totally ignored. The Court stated that although some of the studies suggest that the death penalty may not function as a significantly greater deterrent than less penalties, there is no convincing empirical evidence either supporting or refuting this view.¹⁷ Zeisel⁵² took issue with the Court's statement arguing that the evidence about the deterrent effect is, indeed, "quite sufficient" and that the request for more proof is but "the expression of an unwillingness to abandon an ancient prejudice".

Because of the nature of Ehrlich's claims and the publicity they received there were several replications of his study. The replications revealed fundamental weaknesses in his assumptions, his model and his analysis. His detractors claimed that his evidence of deterrence depends upon a restrictive assumption about the mathematical relationship between homicides and executions,⁶,¹⁸,²⁸ the inclusion of a particular set of observations, the use of a limited set of control variables, and a peculiar construction of the execution rate, the key variable.

One important criticism of Ehrlich's study is his use of time series data for 1933-1969 in which homicides and executions were aggregated for the entire U.S. The Panel of the National Academy of Science⁵⁰ pointed out that Ehrlich's findings were particularly sensitive to the time period included. This sensitivity was largely due to the fact that during 1962-1969, executions in the

U.S. ceased while homicides increased though not more than did other crimes. When Bowers and Pierce⁶ reproduced Ehrlich's analysis using data from slightly different periods, all beginning in 1935 but each ending in a different year in the 1960's, their findings were entirely different from those of Ehrlich. They concluded that:

"It becomes evident that the so-called deterrent effect of execution risk altogether disappears when the effective time period is foreshortened by dropping recent data points."

Another researcher, Passell²⁷ used cross-sectional data in various states in the U.S. from the period 1950 and 1960. He also concluded that there was no reasonable way of interpreting the cross-sectional data that would lend support to the deterrence hypothesis.

A third researcher, Forst¹⁷ replicated Ehrlich's analysis while avoiding some of the major flaws that were identified in Ehrlich's research. For instance, he focused on a unique decade during which the homicide rate increased by over 50% and the use of the death penalty ceased. He examined, as well, changes in homicides and executions over time and across states. His findings did not support Ehrlich's claim that capital punishment deters homicides. His final conclusion:

"The results of this analysis suggest . . . that it is erroneous to view capital punishment as a means of reducing the homicide rate".

Probably the most detailed critique of Ehrlich's research was published in Canada.²¹ Hahn is very conscious of the serious policy implications defective studies might have and is particularly perturbed by the great publicity Ehrlich's study received. His final comments contain both a warning and an advice:

"The techniques introduced by the economists may represent significant advances over those used in the past. These techniques and the behavioural models used by economists are, however, useless unless they are combined with sufficient, accurate and relevant data. Unfortunately, it will be many years before data of sufficient quality and quantity is available for undertaking research which is adequate for supporting deterrence policy.

Until that time, uncritical publicity of the earlier economic findings, publicity that has in the past bordered on the irresponsible, should cease. Economists like Ehrlich should also exercise more professional responsibility in undertaking and reporting what amounts to circumstantial results".