In the previous two death-penalty posts (1, 2), I talked about whether it makes sense to think that the death penalty deters people from killing, given how rarely it's actually applied. John Donohue and Justin Wolfers say this is implausible (PDF). As I explained, I actually think it's pretty consistent with human nature.
But most of their attacks on the recent spate of studies showing the death penalty to be a deterrent are empirical: they say the data just don't support the claim. That gets to the other main problem I have with the Donohue and Wolfers paper:
They compare non-death-penalty states with all death-penalty states, and claim that there are no significant differences between those two types of states in how much they deter homicide. But that seems to be a highly distorted picture of the real situation.
Here's the problem: there are some states that have the death penalty but rarely if ever use it. For instance, several death penalty states have had only one execution each in 40 years. As another example, California executed only 10 people in several decades even though it's the most populous state.
I don't know as much as Donohue and Wolfers know about how to put together an impressive-looking statistical chart, but I know it doesn't make sense to lump together those states with states that regularly execute people.
But is there a way around that? Yes — just break down the death-penalty states into further categories based on how much (and how quickly) they use the death penalty. Joanna Shepherd did just that (PDF), and she found a huge difference among the different kinds of death-penalty states.
In a nutshell, the difference is that only the states that apply the death penalty on a regular basis will achieve the deterrent effect. And those are a small minority of states. The death-penalty states that don't use it much actually have the opposite effect: homicide goes up.
As Shepherd puts it: "On average, an execution in the United States deters crime. [But] these averages are powered by a handful of high-execution, high-deterrence states."
So, if she's right, then that's simultaneously (a) pretty embarrassing for most death-penalty states -- they're actually driving up homicide, but (b) a ringing endorsement of the death penalty itself, as long as it's used right. It basically means that most states would be wise to ramp up their use of the death penalty so that it passes the "threshold" level of death sentences and executions that must be crossed before the death penalty becomes effective.
The New York Times highlighted Shepherd's point about the necessary "threshold" for the death penalty to be an effective deterrent:
According to roughly a dozen recent studies, executions save lives. For each inmate put to death, the studies say, 3 to 18 murders are prevented.This conclusion strikes me as intuitively plausible. But wait a minute -- wasn't I just speculating in a recent post that people who are deciding whether to commit homicide probably realize only that there is the death penalty in their state but are not thinking about the statistical likelihood of being executed? How can I say that, but then turn around and say that the frequency with which a state uses the death penalty does affect people's incentives?
The effect is most pronounced, according to some studies, in Texas and other states that execute condemned inmates relatively often and relatively quickly.
Well, there are good reasons to think people don't run through the whole calculation to figure out the exact percentage of being executed if you're caught. For instance, a Texan might have no idea if the chances are 5% or 1% or what. But at least they know there's a real chance. In many death-penalty states, there isn't even a real chance -- it's basically 0%. If you were living in New Jersey and you knew anything about the death penalty there (before it was recently abolished), you'd know that your state had the-death-penalty-but-not-really: the last execution was in 1963. I find it very plausible that that would negate any deterrent effect, while a state like Texas or Virginia would exert a strong deterrent effect (even beyond what would be rationally justified based on a sober assessment of the actual risk of being executed).
In other words, it seems plausible that there would be some minimal threshold of executions that must be crossed for the death penalty to even register with people as something their state uses at all. But once the state passes that threshold, human nature will cause people to mentally inflate the risk of getting the death penalty.
Back to the studies: I have to give Donohue and Wolfers some credit: they do acknowledge the finding that strong-death-penalty states are the ones with a deterrent effect. But then they utterly dismiss it! They say that if you take Texas — which has executed far more people than any other state -- out of the equation, the deterrent effect pretty much goes away. Well, gee, what direction does that argue for? That certainly seems to mean that the way to deter homicide is to do what Texas does: vigorously apply the death penalty instead of just keeping it on the books without using it. But Donohue and Wolfers somehow see it as noise that's getting in the way of studying the deterrent effect of the death penalty.
The idea that Texas is noise that might be better left out, rather than one of the strongest indicators of the effects of the death penalty, seems like such a blatant mistake that I kept thinking: "Wait, I can't be right about that -- one of the authors is a Yale law professor, and this article was published in the Stanford Law Review. I must be missing something." Well, Cass Sunstein is a University of Chicago law professor, and his article was also published in the Stanford Law Review. And he says it makes no sense to see Texas as noise that creates a deceptive appearance of a deterrent effect:
States having the largest numbers of executions are most likely to deter, and it does not seem to make sense to exclude those states as “outliers.” By way of comparison, imagine a study attempting to determine what characteristics of baseball teams most increase the chance of winning the World Series. Imagine also a criticism of the study ... which complained that data about the New York Yankees should be thrown out, on the ground that the Yankees have won so many times as to be “outliers.” This would be an odd idea, because empiricists must go here the evidence is; in the case of capital punishment, the outliers provide much of the relevant evidence.For all the surface complexity and nuance of the Donohue and Wolfers paper, they seem to have drastically oversimplified and distorted the situation.
By the way, there have also been two other academic articles specifically devoted to disproving Donohue and Wolfers's claims: 1, 2. (Those links go to the abstracts, but you can download the full PDF for free by clicking "one-click download.")
Again, I'm not really qualified to judge Donohue and Wolfers's study, but from what I can tell, it seems like a very weak rejoinder to the abundance of new research suggesting that the death penalty is indeed a deterrent.
Now, even if you're convinced by all that, I could understand saying that we should err on the side of not actively killing people if the data are even debatable. But I'm not so sure. We're never going to have definitive proof of the death penalty's deterrent effect or lack thereof. From what I can tell, the stronger argument is that it is a deterrent. You can't make policy with perfect knowledge. You can only make the best possible estimate, and act on that.