In 2005, there were 16,692 cases of murder and non-negligent manslaughter nationally. There were 60 executions.
The studies' conclusions drew a philosophical response from a well-known liberal law professor, University of Chicago's Cass Sunstein. A critic of the death penalty, in 2005 he co-authored a paper titled "Is capital punishment morally required?"
"If it's the case that executing murderers prevents the execution of innocents by murderers, then the moral evaluation is not simple," he told The Associated Press. "Abolitionists or others, like me, who are skeptical about the death penalty haven't given adequate consideration to the possibility that innocent life is saved by the death penalty."
Sunstein said that moral questions aside, the data needs more study.
Critics of the findings have been vociferous.
Some claim that the pro-deterrent studies made profound mistakes in their methodology, so their results are untrustworthy. Another critic argues that the studies wrongly count all homicides, rather than just those homicides where a conviction could bring the death penalty. And several argue that there are simply too few executions each year in the United States to make a judgment.
"We just don't have enough data to say anything," said Justin Wolfers, an economist at the Wharton School of Business who last year co-authored a sweeping critique of several studies, and said they were "flimsy" and appeared in "second-tier journals."
"This isn't left vs. right. This is a nerdy statistician saying it's too hard to tell," Wolfers said. "Within the advocacy community and legal scholars who are not as statistically adept, they will tell you it's still an open question. Among the small number of economists at leading universities whose bread and butter is statistical analysis, the argument is finished."
Several authors of the pro-deterrent reports said they welcome criticism in the interests of science, but said their work is being attacked by opponents of capital punishment for their findings, not their flaws.
"Instead of people sitting down and saying 'let's see what the data shows,' it's people sitting down and saying 'let's show this is wrong,"' said Paul Rubin, an economist and co-author of an Emory University study. "Some scientists are out seeking the truth, and some of them have a position they would like to defend."
The latest arguments replay a 1970s debate that had an impact far beyond academic circles.
Then, economist Isaac Ehrlich had also concluded that executions deterred future crimes. His 1975 report was the subject of mainstream news articles and public debate, and was cited in papers before the U.S. Supreme Court arguing for a reversal of the top U.S. court's 1972 suspension of executions. (The court, in 1976, reinstated the death penalty.)
Ultimately, a panel was set up by the National Academy of Sciences which decided that Ehrlich's conclusions were flawed. But the new pro-deterrent studies have not gotten that kind of scrutiny.
At least not yet. The academic debate, and the larger national argument about the death penalty itself — with questions about racial and economic disparities in its implementation — shows no signs of fading away.
Steven Shavell, a professor of law and economics at Harvard Law School and co-editor-in-chief of the American Law and Economics Review, said in an e-mail exchange that his journal intends to publish several articles on the statistical studies on deterrence in an upcoming issue.