Some more data on the supposed deterrent effect of capital punishment:
In 2003, there were [in the U.S.] 16,503 homicides (including nonnegligent manslaughter), but only 144 inmates were sentenced to death. Moreover, of the 3374 inmates on death row at the beginning of the year, only 65 were executed. Thus, not only did very few homicides lead to a death sentence, but the prospect of execution did not greatly affect the life expectancy of death row inmates. Indeed, Katz, Levitt, and Shustorovich have made this point quite directly, arguing that “the execution rate on death row is only twice the death rate from accidents and violence among all American men” and that the death rate on death row is plausibly lower than the death rate of violent criminals not on death row. As such they conclude that “it is hard to believe that in modern America the fear of execution would be a driving force in a rational criminal’s calculus.” John J. Donohue III and Justin Wolfers (source)
Proponents of capital punishment may answer this in two ways:
1. It proves their point: if all these data are correct, we need more capital punishment, and then the deterrent effect will kick in. Capital punishment as it is used now may indeed not deter significantly, but that’s no reason to abolish it; it’s a reason to step up the production of corpses.
But this reasoning leads to a reductio ad absurdum: if deterring crime is so important, and if we should do more to deter crime, then why don’t we change the execution methods: burn criminals alive at the stake. That should deter. But this, of course, brings home the point that we simply can’t do what we want to people in order to achieve some beneficial aggregate social good. If proponents of the death penalty shy away from this ultimate implication of the deterrent argument – and I think most of them will – then there’s no reason why opponents cannot have good reasons to reject killing criminals in other, less cruel ways. If propopents concede the point that there are certain things we can’t do to people, regardless of who they are or what they’ve done, then opponents can make the case that these “certain things” do not only include burning people alive but also killing them in a way which is less cruel but which nevertheless implies instrumentalizing people for the benefit of others with whom they have no relationship and who may not have been born yet. This instrumentalization is perhaps not physically cruel, but it is dehumanizing. People are no longer viewed as humans but as tools for the maximization of social wellbeing.
2. The calculating criminal is a myth. Murderers don’t look at death row statistics or other statistics mentioned in the quote above in order to decide whether or not to actually kill someone. They are deterred, not by numbers, but by the general vivid image of the horror of capital punishment. That may be true in the case of some types of murderers (e.g. the uneducated ones, or those motivated by passion), but not in the case of other types (some people may indeed look at the data and calculate that the risk of being killed for their crimes is so low that it’s ok to go ahead*).
But even if it is true and people don’t calculate, the “burning at the stake” implication still holds. If it’s the vivid nature of the punishment that counts as a deterrent, not the statistical likelihood of actually receiving this punishment (which is very low as a matter of fact), then let’s make it as “vivid” as possible and bring back the Middle Ages.
* I’m thinking of professional criminals for example.