Becker has presented in his post today a compelling restatement of the economic case for capital punishment. I have a few minor disagreements and qualifications, and I will first mention them and then respond to some of the very large number of comments that my last week's posting elicited.
I do not consider revenge an impermissible ground for capital punishment. Revenge has very deep roots in the human psyche. As I have long argued, basing the argument on work by evolutionary psychologists such as Robert Trivers, the threat of revenge must have played an essential role in maintaining order in the "ancestral environment." That is a term that evolutionary biologists use to describe the prehistoric era in which human beings evolved to approximately their current biological state. In that era there were no laws, police, etc., so the indignation that would incite a person to revenge himself upon an aggressor must have had substantial survival value and become "hard wired" in our brains. The wiring remains, and explains some of the indignation that people feel, especially but not only the friends and family members of murder victims, toward the murderer. It seems plausible to me (here modifying what I said in my original posting) that the net increment in utility that they derive from the execution (versus life imprisonment) of the murderer exceeds the net increment in disutility that the murderer derives from being executed rather than imprisoned for life. The strong support for capital punishment in public opinion polls provides limited support for this conjecture.
I do not favor public executions; nor dismemberment or other horrific modes of execution. The incremental deterrent effect might well be nontrivial, but would be outweighed by public revulsion. There is also the danger of brutalization. As Friedrich Nietzsche pointed out, making people squeamish is one of the projects of modernity, and may explain the banning of blood sports as well as the movement away from public and gruesome executions. The idea is that if people become unaccustomed to bloody sights they will be less likely to employ violence in their relations with other people. Still another objection to public and gruesome executions is that they offer murderers an opportunity to die as heroes by showing fortitude.
I agree that marginal deterrence is important and that it generally argues for reserving the heaviest sentences for the most serious crimes. But there are two important qualifications. First, a very heavy sentence may be necessary to deter a minor crime because the likelihood of apprehension is very low. The expected punishment cost of crime is, as a first approximation (ignoring attitude toward risk), the punishment if imposed multiplied by the probability of imposition, so if the probability is very low a compensating increase in punishment is indicated. This does not impair marginal deterrence as long as the crimes are not close substitutes: a heavy fine for litterers will not increase the robbery rate, whereas capital punishment for robbers would increase the murder rate (of robbers' victims)--were it not for my second qualification. Even if murder and robbery were both capital crimes, there would be marginal deterrence because the police would search much harder for a robber who murdered his victim; the more extensive search would compensate, in part anyway, for the loss of the information that the victim could have given the police to identify the robber. Moreover, capital punishment is merely a ceiling; even if robbery were a capital crime, judges and juries would be much less likely to impose the death sentence on a robber who had not killed his victim than on one who had.
Marginal-deterrence theory provides, however, a compelling reason to execute prisoners sentenced to life without parole who murder in prison; the threat of a sentence of imprisonment can have no deterrent effect on them.
Becker mentions the possibility of racial discrimination in execution. Studies done some years ago--I do not know whether they would be descriptive of current practice--revealed the following pattern: murderers of black people were less likely to be executed than murderers of white people. Since blacks were more likely to murder other blacks than to murder whites, this meant that blacks were less rather than more likely to be executed than whites, relative to the respective murder rates of the two races. (Blacks commit murders at a much higher rate than whites.) The explanation offered was that judges and juries tended to set a lower value on black victims of murder than on white ones. From this some observers inferred that capital punishment discriminates against blacks. The inference is incorrect. The proper inference is that murderers of blacks are underpunished.
I turn now to the comments on my posting. A long comment by "ohwilleke" makes a number of interesting points, but they do not support his opposition to capital punishment. He notes first of all that many factors influence the murder rate besides the probability of execution. That is true, but it does not, as he suggests, make it "insanely difficult to make any econometric estimate" that is not "meaningless." Econometrics, which is to say the set of statistical methods used by economists to try to tease out causal factors, enables the particular factor of interest, in this case the probability of execution, to be isolated. The methods are not entirely reliable, which is why neither Becker nor I claim that economists have proved that capital punishment deters; we merely claim that there is significant evidence that it does. I note how many commenters remark correctly that murder rates are higher in the South, even though that is where most executions occur, than in other regions. But that is not an argument that executions do not deter. The higher the background rate of murder, the more severe one expects punishments to be. A high murder rate implies a high expected benefit from murder and so the expected cost of punsihment has to be jacked up to offset that greater benefit.
Ohwilleke's comment claims that the "error rate" in capital punishment is 10 percent. This is incorrect. Not a single person among the 119 that he contends were erroneously sentenced to death was executed. That is a zero error rate.
One commenter asks whether capital punishment "really deter[s] the type of person who actually does the murdering?" Of course if someone in a state that has capital punishment commits the kind of murder that puts him at risk of such punishment, the threat of capital punishment has not deterred. This is the usual situation with criminal punishment. People who commit crimes are people for whom the expected cost of punishment, combined with the other costs of crime, is less than the expected benefit of the crime. The purpose of punishing these people is not to deter them--by committing the crime in the face of threatened punishment they have shown themselves to be undeterrable--but to deter people who, were it not for the expected punishment cost, would commit the crime because its other costs were lower than its expected benefit.
Finally, several comments usefully point out that capital punishment has a secondary deterrent effect: it induces murderers to plead guilty and receive a life sentence.