Scholars who tried to assess the preventive functions of the death penalty used various methods to test the deterrence hypothesis. And proponents and opponents of the death penalty used various types of evidence to support or to challenge its unique deterrent effect. This evidence may be divided into two main categories: anecdotal stories and statistical findings.

Proponents of the death penalty usually argue that almost every prisoner under sentence of death seeks a reprieve and welcomes it when it comes. This is seen as evidence that men fear death more than anything else and far more than life imprisonment. It seems fallacious to assume from the terror of death experienced and manifested by an individual on death row that the same fear was operative in his mind at the time of the crime. This argument overlooks one indisputable fact: the difference between a potential and remote danger and one that is imminent and seemingly inevitable. In Sellin's words:

"Surely a murderer, for whom a possible death penalty had proved to be no deterrent, would be considered abnormal were he not to make every effort to escape death after being discovered and sentenced to die".⁴¹

Another fact this argument overlooks is that the murderer on death row who is showing extreme fear and terror in face of execution has not been deterred by the threat of death in the first place. It seems illogical to use the words or the psychological state of those who were not deterred by the death penalty to prove this penalty's unique deterrent effect!

Proponents of the death penalty also cite real life stories of criminals who have told the police that they refrained from killing the victim or from shooting at the pursuing police officer to avoid being put to death. For example, the Los Angeles Police Department reported to a California Senate Committee considering the abolition of the death penalty⁴⁷ that during the course of one year, 13 robbery suspects had told the police that they used unloaded or simulated guns "rather than take a chance on killing someone and getting the gas chamber". The unreliability of such anecdotical evidence is too obvious and for every story alleging that the fear of the death penalty has acted as a deterrent there are ten others alleging that it has not. Clinton Duffy former warden of San Quentin prison asked thousands of prisoners convicted of homicide or armed robbery whether they had thought of the death penalty before their act. Not one had!Among the most frequently quoted stories to deny the deterrent effect of capital punishment are those of the English pickpockets who actively plied their trade in the shadow of the gallows from which their fellow knaves were strung. Another often cited story is the one of an Ohio convict named Charlie Justice who devised the clamps that held the condemned man in the electric chair. After his release, he was convicted of murder and electrocuted. A similar fate befell Alfred Wells who helped install San Quentin's gas chamber in 1938. It was his conversational cachet around the prison yard, usually with the moral: "That's the closest I ever want to come. . .". Four years later, back at San Quentin for a triple killing, he was sealed in the chamber to die.Needless to say that arguments and claims based on this kind of anecdotal evidence tend to neutralize each other and are of little help in settling the basic factual question whether or not the death penalty is a unique deterrent.

Earlier studies on the preventive functions of the death penalty tried to ascertain its deterrent effect in five different ways: