What gets little notice, however, is a series of academic studies over the last half-dozen years that claim to settle a once hotly debated argument–whether the death penalty acts as a deterrent to murder. The analyses say yes. They count between three and 18 lives that would be saved by the execution of each convicted killer.
However, there is little new since the last time I wrote about Donohue and Wolfers’ take down of those studies. They found that the results of those studies were not robust — simple changes to the models made the effect go away. Here’s an interesting graph from Donohue and Wolfers:
It shows the size of the effect (vertical axis) versus its standard error. The interesting thing is that there is a strong relation between these two numbers. Studies that have large standard errors also find a large effect. Which is just as well, because if they didn’t find a large effect, the result would not be statistically significant. This is evidence for publication bias. Even if there is no effect, by chance some specifications will be significant at the 5% level and these are the ones that get published. In this case, the published results will tend to be barely significant, and the graph shows that as well.
Steven Levitt comments:
Given the evidence I’ve examined, I believe that Wolfers is on the right side of this debate. There are recent studies of the death penalty — most bad, but some reasonable — that find it has a deterrent effect on crime. Wolfers and John Donohue published an article in the Stanford Law Review two years ago that decimated most of the research on the subject.
Analyses of data that stretches further back in time, when there were many more executions and thus more opportunities to test the hypothesis, are far less charitable to death penalty advocates. On top of that, as we wrote in Freakonomics, if you do back-of-the-envelope calculations, it becomes clear that no rational criminal should be deterred by the death penalty, since the punishment is too distant and too unlikely to merit much attention. As such, economists who argue that the death penalty works are put in the uncomfortable position of having to argue that criminals are irrationally overreacting when they are deterred by it.