The other week George Will repeated his frequent claim that the simple correlation of crime rates and jail rates tells you something–that harsh jail sentences reduces crime.  One would have to be a fool, he alleges, to wonder whether that were the case.  With that in mind, it's interesting to read Cass Sunstein and Justin Wolfers (actual law professors) on the deterrent effect of the death penalty.  Their conclusion (after what appears to be actual research): dunno.  Here's a selection:

One might like to conclude that these latter studies demonstrate that the death penalty does not deter. But this is asking too much of the data. The number of homicides is so large, and varies so much year to year, that it is impossible to disentangle the effects of execution policy from other changes affecting murder rates. Moreover, execution policy doesn't change often or much. Just as a laboratory scientist with too few experimental subjects cannot draw strong conclusions, the best we can say is that homicide rates are not closely associated with capital punishment. On the basis of existing evidence, it is especially hard to justify claims about causality.

Justice Stevens argues, "In the absence of such evidence, deterrence cannot serve as a sufficient penological justification for this uniquely severe and irrevocable punishment." Perhaps. But the absence of evidence of deterrence should not be confused with evidence of absence.

Justice Scalia relies on the suggestion by Sunstein and Vermeule that some evidence suggests a possible deterrent effect. But that suggestion actually catalyzed Donohue and Wolfers's study of available empirical evidence. Existing studies contain significant statistical errors, and slightly different approaches yield widely varying findings, a problem exacerbated by researchers' tendency to report only those results supporting their conclusions. This led Sunstein and Vermeule to acknowledge: "We do not know whether deterrence has been shown. . . . Nor do we conclude that the evidence of deterrence has reached some threshold of reliability that permits or requires government action."

In short, the best reading of the accumulated data is that they do not establish a deterrent effect of the death penalty.

It seems to be obvious that Stevens believes the burden of proof lies with those who assert the causal connection.  In the absence of such evidence for their claim, they can't make the assertion.  Showing that it's not the case might be akin to making the accused prove that he's innocent.  For, after all, there's always the possibility that there is a correlation we haven't discovered yet (and the claims of astrology might also turn ought to be true).  Disproving such a connection would be very difficult and it's silly that Sunstein and Wolfers would suggest this a reasonable request–especially in an op-ed about causal connections for which they claim no positive evidence exists.

The absence of such a correlation, of course, might be seen as a separate question from whether the death penalty is justified (but they don't argue this).  If one's justification for capital punishment relies on deterrence, then the answer is obviously no.  They write:

Why is the Supreme Court debating deterrence? A prominent line of reasoning, endorsed by several justices, holds that if capital punishment fails to deter crime, it serves no useful purpose and hence is cruel and unusual, violating the Eighth Amendment. This reasoning tracks public debate as well. While some favor the death penalty on retributive grounds, many others (including President Bush) argue that the only sound reason for capital punishment is to deter murder.

We concur with Scalia that if a strong deterrent effect could be demonstrated, a plausible argument could be made on behalf of executions. But what if the evidence is inconclusive?

We are not sure how to answer that question. But as executions resume, the debates over the death penalty should not be distorted by a misunderstanding of what the evidence actually shows.

This is baffling.  While the authors deny positive evidence for deterrence, they fail to make the point that there might be some independent justification for capital punishment, like punishment.  Instead they retreat into an absurd hypothetical–if it does deter crime, then yes.  But there's no evidence that it does, so the reasonable conclusion would be that it's not justified on that basis, would it not?