Justin Wolfers and Cass Sunstein say members of the Supreme Court "misread the evidence" on the deterrent effect of the death penalty:
A Death Penalty Puzzle The Murky Evidence for and Against Deterrence, by Cass R. Sunstein and Justin Wolfers, Commentary, Washington Post: ...Last month, capital punishment resumed after a seven-month moratorium. Rapid scheduling of executions followed the Supreme Court's ruling in Baze v. Rees, reaffirming the constitutionality of the death penalty in general and lethal injection in particular.
To support their competing conclusions on the legal issue, different members of the court invoked work by each of us on the deterrent effects of the death penalty. Unfortunately, they misread the evidence.
Justice John Paul Stevens cited recent research by Wolfers (with co-author John Donohue) to justify the claim that "there remains no reliable statistical evidence that capital punishment in fact deters potential offenders." Justice Antonin Scalia cited a suggestion by Sunstein (with co-author Adrian Vermeule) that "a significant body of recent evidence" shows "that capital punishment may well have a deterrent effect, possibly a quite powerful one."
What does the evidence actually say? ...
In short, the best reading of the accumulated data is that they do not establish a deterrent effect of the death penalty.
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. ...
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.