Via WaPo by Cass Sunstein and Justin Wolfers:
One might like to conclude that these latter studies [that show that the death penalyy is not correlated with homicide/crime] 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.
... "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.
And on the repeal of the DC handgun law in the NYT:
But Gary Kleck, a professor at Florida State University’s College of Criminology and Criminal Justice, whose work Justice Breyer cited, said there were good reasons for making a definitive judgment.
“We know the D.C. handgun ban didn’t reduce homicide,” he said in an interview.
On guns, I'm a little more concerned with the focus on homicide versus accidental death, suicide, assault with deadly weapon, battery, spousal assaults, etc. that do not fall into the homicide statistics.