NEW YORK - For the first time in a generation, the question of whether the death penalty deters murders has captured the attention of scholars in law and economics, setting off an intense new debate about one of the central justifications for capital punishment.
According to roughly a dozen recent studies, executions save lives. For each inmate put to death, the studies say, three to 18 murders are prevented.
The effect is most pronounced, according to some studies, in Texas and other states that execute condemned inmates relatively often and relatively quickly.
The studies, performed by economists in the past decade, compare the number of executions in different jurisdictions with homicide rates over time - while trying to eliminate the effects of crime rates, conviction rates, and other factors - and say that murder rates tend to fall as executions rise. One study looked at 3,054 counties over two decades.
"I personally am opposed to the death penalty," said H. Naci Mocan, an economist at Louisiana State University and an author of a study finding that each execution saves five lives. "But my research shows that there is a deterrent effect."
The studies have been the subject of sharp criticism, much of it from legal scholars who say that the theories of economists do not apply to the violent world of crime and punishment.
The death penalty "is applied so rarely that the number of homicides it can plausibly have caused or deterred cannot reliably be disentangled from the large year-to-year changes in the homicide rate caused by other factors," John J. Donohue III, a law professor at Yale with a doctorate in economics, and Justin Wolfers, an economist at the University of Pennsylvania, wrote in the Stanford Law Review in 2005. "The existing evidence for deterrence," they concluded, "is surprisingly fragile."
Gary Becker, who won the Nobel Prize in economics in 1992 and has followed the debate, said the current empirical evidence was "certainly not decisive" because "we just don't get enough variation to be confident we have isolated a deterrent effect."
But, Becker added, "the evidence of a variety of types - not simply the quantitative evidence - has been enough to convince me that capital punishment does deter and is worth using for the worst sorts of offenses."
The debate, which first gained significant academic attention two years ago, reprises one from the 1970s, when early and since largely discredited studies on the deterrent effect of capital punishment were discussed in the Supreme Court's decision to reinstitute capital punishment in 1976 after a four-year moratorium.
The early studies were inconclusive, Justice Potter Stewart wrote for three justices in the majority in that decision. But he nonetheless concluded that "the death penalty undoubtedly is a significant deterrent."
But the studies have started to reshape the debate over capital punishment and to influence prominent legal scholars.
"The evidence on whether it has a significant deterrent effect seems sufficiently plausible that the moral issue becomes a difficult one," said Cass R. Sunstein, a law professor at the University of Chicago who has frequently taken liberal positions.
Sunstein and Adrian Vermeule, a law professor at Harvard, wrote in their own Stanford Law Review article that "the recent evidence of a deterrent effect from capital punishment seems impressive, especially in light of its 'apparent power and unanimity,' " quoting a conclusion of a separate overview of the evidence in 2005 by Robert Weisberg, a law professor at Stanford, in the Annual Review of Law and Social Science.
To a large extent, the participants in the debate talk past one another because they work in different disciplines.
To many economists, it follows inexorably that there will be fewer murders as the likelihood of execution rises.
"I am definitely against the death penalty on lots of different grounds," said Joanna M. Shepherd, a law professor at Emory University with a doctorate in economics who wrote or contributed to several studies. "But I do believe that people respond to incentives."
But not everyone agrees that potential murderers know enough or can think clearly enough to make rational calculations.
And the chances of being caught, convicted, sentenced to death, and executed are in any event quite remote. Only about one in 300 homicides results in an execution.