If capital punishment deterred the most awful crimes any better than decades of imprisonment, that would be an important fact. A century of research has failed to produce convincing evidence that it does. Comparing murder rates in states before and after the death penalty was reintroduced can't filter out other influences on crime, such as changes in demographic and economic conditions. Comparing states with and without capital punishment is also inconclusive; in death-penalty states, capital crimes can be punished by long terms in prison instead, so their respective effects can't be untangled.
How likely is it, really, that a killer will be more deterred by the risk of the death penalty than by having to spend the rest of his life in prison? The claim fails the test of common sense. Criminologists and police chiefs say the death penalty just doesn't influence murderers partly because its application is so haphazard.
This arbitrariness, of course, is a gross injustice in its own right. As well as being confined to people who live in certain states, the death penalty has been imposed disproportionately on the poor and uneducated, on defendants with substandard lawyers, and on those whose victims were white. A study in Maryland found that a black killer of a white victim was 11 times more likely to be sentenced to death than a white killer of a black victim. These disparities violate the constitutional guarantee of equal protection of the law.
Worst of all, execution risks imposing the ultimate and irrevocable punishment on the wrong person. The 18th century English jurist William Blackstone wrote that it is better that 10 guilty people escape than that one innocent suffer. A system that accepts any risk, however small, of putting the innocent to death should provoke special revulsion.
Whether this has actually happened is up for debate: There's no way for an executed person to be officially exonerated. Yet serious doubts have been raised in a dozen cases just in the past decade. In the past 40 years, more than 130 people have been released from death row after evidence emerged of their wrongful convictions. That number is almost certain to grow because more convictions are being challenged on grounds of DNA evidence, police and prosecutorial misconduct, and the increasingly acknowledged limitations of eyewitness testimony.
It's true that the purpose of punishment is not only deterrence but also retribution. But this doesn't justify the popular view that killers should be killed, any more than it would support the idea that rapists should be raped or thieves stolen from. To be just, retribution must be measured and restrained. That's the difference between justice and revenge.
What about those cases in which the crime is truly monstrous and there can be no doubt about guilt? Timothy McVeigh, the terrorist who murdered 168 people in the 1995 Oklahoma City bombing, comes to mind. For that matter, think of Adolf Hitler or Pol Pot.
Most abolitionists argue that capital punishment would be morally wrong even in these egregious cases, because it is evil in itself, and because the state has no right to commit premeditated killing. This goes too far. The state does have that right, and exercises it frequently for purposes of national defense. In principle, the death penalty might indeed be just punishment for those most heinous crimes.
The problem is that this principle can't be safely applied, certainly not as a matter of routine criminal justice. The extraordinary crimes that would justify the death penalty are difficult to imagine, much less define, before the fact. And, even in exceptional cases, the requirement to prove guilt beyond any doubt is hard to satisfy. (What does "beyond any doubt" actually mean? Is a psychopath guilty beyond any doubt?)
Let's allow that it would have been right to execute Hitler. But let's also recognize that restricting the death penalty to the few cases where it would be both just and safe is impractical. The best pragmatic course is not to use the death penalty more sparingly but to abolish it outright.