Update: added missing caption to figure
Next year, the New Hampshire House will take up a bill to abolish the death penalty. Several libertarian legislators have signed on as co-sponsors, and observers think the bill has a good chance.
What should libertarians think about the death penalty? In general, hardcore civil libertarians have opposed it, but there does not seem to be anything in the moral principles libertarians have adopted that straightforwardly generate an a priori skepticism of capital punishment. The case for or against capital punishment depends on empirical research in a way that the case, say, for or against certain gun laws does not. (Banning handguns would be wrong even if it reduced the violent crime rate, I would argue.)
Some libertarians say that the state can never be trusted with the death penalty. But this too is an empirical claim. What is the rate of killing of innocents when the government has a death penalty of a certain kind? We also need to think about what rate would be unacceptable. (Every criminal justice system will punish some innocents, because no criminal justice system is perfect.) That latter threshold presumably depends on what the deterrent effect of capital punishment is. If capital punishment deters a significant number of murders, presumably an extremely rare execution of an innocent — while horrible and thoroughly regrettable — does not make the system unacceptable.
So does capital punishment deter murders? Apparently not. The literature on capital punishment in the U.S. has shown mixed results, with some models showing a positive deterrent effect (lives saved) and others showing a negative “deterrent” effect (more murders). The recent research by Durlauf, Fu, and Navarro (here and here) helps us adjudicate among these results.
The most plausible set of models consists of those that assume that a higher probability of execution affects murder rate via a logistic function (which reduces the influence outliers). Here’s how they summarize their results in the Journal of Quantitative Criminology for various sets of models (marginal effects reported for the mean 1996 death penalty state):
Fig 2: Positive deterrent effect of executions (net lives saved)
The “linear, state coefficients” models are the least plausible: these models assume the deterrent effect of the death penalty (that is, the marginal effect of executions on the decision to commit homicide) varies by state. As they point out, that is a bit like assuming that the treatment effect of a drug is different in Texas than in other states. In general, linear models are less plausible than logistic models, which assume a functional form more appropriate to the data.
All of the logistic models show a net lives lost effect from capital punishment. Varying the other model details seem not to make a big difference to the results. However, the authors also calculate the posterior probability of the model’s being true given its assumptions and the data, and model 16 above comes out best. This model yields one of the highest “negative deterrent” effects of capital punishment.
In summary, the evidence leads me to believe that capital punishment does not, on net, save lives. It may even cost lives through a kind of “brutalization” process. This information is highly relevant to the normative policy implications.
PILEUS 
I would argue that a large portion of any deterrent effect is squandered in the US by the Byzantine appeals process that removes both the immediacy and certain contingency of the punishment thereby eroding its efficacy.
Right, but then the question is whether any politically and judicially feasible death penalty in the U.S. could work differently. If not, we may be better off abolishing it than continuing with what we have.
As Jason says, it is unlikely that a civilised judicial system would ever be able to dispense with a lengthy appeals process. Nor do I think it would be especially desirable, since there is an obvious trade-off between efficiency and minimising the risk of miscarriages of justice. If you are prepared to be ruthlessly utilitarian about it, then maybe you could justify such a system, but otherwise not.
First off, I’m not sure what the graphic’s showing (-70 WHAT?). From your comments about it, I’m guessing it’s trying to predict the likelihood that someone will commit a murder because they’re “afraid” of the death penalty. Of all the arguments for and against the death penalty, I think that’s the weakest. The likelihood of getting caught is downplayed in a murderer’s mind, so much more so the type of punishment.
For me, the death penalty boils down to three things: the likelihood of recividism, the economic costs of incarceration vs. execution, and the chance of executing someone innocent of the crime.
Recidivism: If they’re dead, they can’t commit again. If they’re put away for X amount of time, they might commit again when they get out. Even if they’re put away for life, there’s a chance they could escape. So this is an argument for capitol punishment, but I’d only apply it if the nature of the crime suggests it’s repeatable (A serial killer will be going back to the same environment as where he killed before, a battered wife that killed her cheating husband probably won’t be a danger to the general public.)
Economics: This can go either way, depending on how much it costs to execute someone and whether they do any useful work while they’re incarcerated. I think it usually ends up being cheaper to kill them, though I may be biased, since I’m pro-“check the population explosion.” it’s probably the strongest reason that I (and I count myself a libertarian) have for the death penalty.
Innocence: The more 20/20 and Dateline I watch, the more I realize how much maneuvering is involved as to what evidence is and is not heard. So I’m not surprised about the number of people serving murder convictions that have plausible arguments about why they’re innocent. Financial recompense may seem inadequate for many years of incarceration, but there’s no way to give anything back when a life is taken in error. This, I think, is the best argument against the death penalty.
So there you have it: three arguments in a nutshell, and none of them even considering the deterrent effects.
Sorry, I just realized the caption for the figure was not part of the figure itself. I’ve since added it.
“The case for or against capital punishment depends on empirical research in a way that the case, say, for or against certain gun laws does not. (Banning handguns would be wrong even if it reduced the violent crime rate, I would argue.)”
There would appear to be a pretty strong non-empirical argument. Given that errors in the justice system cannot possibly eliminated, how can any government respectful of individual liberty utilize irreversible punishments?
If you wrongly imprison someone you can release them and pay compensation. If you execute them…….