In debates on the death penalty, it is common for opponents to state flatly that the death penalty has been proven not to deter and that the studies finding deterrence have been discredited. Baloney. Here is the assessment of Gary Becker, Nobel laureate and the man who pioneered the use of economic methods to study crime, as posted on his blog a few years ago:

Of course, public policy on punishments cannot wait until the evidence is perfect. Even with the limited quantitative evidence available, there are good reasons to believe that capital punishment deters murders. Most people, and murderers in particular, fear death, especially when it follows swiftly and with considerable certainty following the commission of a murder. As Posner indicates, the deterrent effect of capital punishment would be greater if the delays in its implementation were much shortened, and if this punishment was more certain to be used in the appropriate cases. But I agree with Posner that capital punishment has an important deterrent effect even with the way the present system actually operates.

Responding to critical comments on this post,  Prof. Becker followed up:

I stated that the evidence from quantitative studies is decidedly mixed, yet I concluded that "the preponderance of evidence does indicate that capital punishment deters." Although the weight of the positive evidence should not be overstated, the frequently stated claim that these studies prove that capital punishment does not deter is clearly false.

If the death penalty does deter, then repealing it kills innocent people. If it does not, then keeping it simply results in the execution of punishments that are independently justified as "just deserts" for the crime committed. The opponents' claim of nondeterrence is the one less likely to be true by a considerable margin, and they should surely have the burden of proof given that their proposal will cost innocent lives if they are wrong. The opponents who say lack of deterrence has been proven are just flat lying.