The Death Penalty

Andrew Sullivan asks whether California will be the next state to abandon the death penalty. This reminds me of something I noticed a few years ago: capital punishment remains popular—-a clear majority of American support the death penalty for people convicted of murder—-its supporters are on the defensive. Death-penalty opponents, while in the minority, seem to have the upper hand in debates on the issue.

My larger perspective on the death penalty, informed by my research with Jim Liebman several years ago, is that you can only accept capital punishment if you’re willing to have innocent people executed every now and then. And, the more effective you want the death penalty to be, the more innocents you have to execute.

The occasional execution of innocent people might be deemed ok in some settings—-they shoot deserters in wartime, and if a country is in the midst of a big enough crime wave, I could see people accepting the need for the occasional lethal mistake of the judicial process. My point here is just that if you want to execute people on a regular basis, you’re gonna make some mistakes. We saw this in our research on death-sentencing reversals, which were not merely the actions of a few liberal court panels.

Here’s what I wrote a few years ago in my discussion of an excellent paper by Donohue and Wolfers:

Policy questions about the death penalty have sometimes been expressed in terms of the number of lives lost or saved by a given sentencing policy. But I think this direction of thinking might be a dead end. First off, as discussed by Donohue and Wolfers, it may very well be essentially impossible to statistically estimate the net deterrent effect of death sentencing—-what seem like the “hard numbers” (in Richard Posner’s hopeful words, “careful econometric analysis”) aren’t so clear at all.

More generally, though, I’m not sure how you balance out the chance of deterring murders with the chance of executing an innocent person. What if each death sentence deterred 0.1 murder, and 5% of people executed were actually innocent? That’s still a 2:1 ratio (assuming that it’s OK to execute the guilty people). Then again, maybe these innocent people who were executed weren’t so innocent after all. But then again, not every murder victim is innocent either. Conversely, suppose that executing an innocent person were to deter 2 murders (or, conversely, that freeing an innocently-convicted man were to un-deter 2 murders). Then the utility calculus would suggest that it’s actually OK to do it. In general I’m a big fan of probabilistic cost-benefit analyses (see, for example, chapter 22 of Bayesian Data Analysis), but here I don’t see it working out. The main concerns—-on the one hand, worry about out-of-control crime, and on the other hand, worry about executing innocents—-just seem difficult to put on the same scale.

Finally, regarding decision analysis, incentives, and so forth: much of the discussion (not in the Donohoe and Wolfers paper, but elsewhere) seems to go to the incentives of potential murderers. But the death penalty also affects the incentives of judges, juries, prosecutors, and so forth. One of the arguments in favor of the death penalty is that it sends a message that the justice system is serious about prosecuting murders. This message is sent to the population at large, I think, not just to deter potential murderers but to make clear that the system works. Conversely, one argument against the death penalty is that it motivates prosecutors to go after innocent people, and to hide or deny exculpatory evidence. Lots of incentives out there.

[Cross-posted at The Monkey Cage]

Andrew Gelman

Andrew Gelman is a professor of statistics and political science and director of the Applied Statistics Center at Columbia University.