That's the title of an OpEd in today's Washington Post by Cass Sunstein and Justin Wolfers. It's subtitled, "The Murky Evidence for and Against Deterrence."
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?
One approach notes that in states with the death penalty, the average murder rate is about 40 percent higher than in states without the death penalty. Yet such comparisons are surely confounded by other influences, as those states that impose the death penalty also have a historic culture of violence, including lynching.
If we compare countries, the United States has higher execution and higher homicide rates than nearly all other industrialized countries. Here, too, many alternative explanations remain, making it hazardous to conclude that the death penalty does not deter murder.
Other studies have evaluated changes in homicide rates over time. In the 1960s, as the death penalty fell into disuse, homicide rates rose sharply, leading some studies to infer a deterrent effect. Moreover, a large-scale decline in homicide in the past two decades coincided with renewed use of the death penalty. Countering this, homicide and execution rates rose together in the 1920s and early 1930s, then fell together through the 1940s and 1950s. Because conclusions are so sensitive to the time period evaluated, these studies fail to provide much help.
More sophisticated studies compare the evolution of homicide rates across jurisdictions. Over the past six decades, the homicide rate in Canada has tracked that in the United States even as the countries' punishment policies have diverged sharply. Similarly, the 12 states that have not executed a prisoner since 1960 comprise a useful comparison group; murder rates in these states have largely tracked those in states that subsequently adopted or rejected the death penalty.
And:
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.
The deterrence index is here.
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