The National Academy of Sciences picked apart decades of deterrence research last year and recommended "that these studies not be used to inform deliberations" on capital punishment, reported the Denver Post.
University of Colorado at Boulder professor Michael Radelet surveyed 90 leading crime researchers around the nation in 2008, seeking their professional critique of deterrence evidence. Reflecting a previous 1990s study almost exactly, 88 percent of his respondents said they do not believe the death penalty is a deterrent to murder.
Those who say deterrence is impossible to sort out cite these main challenges:
• There's no way to tell if a murderer was aware of a state's death-penalty status when the crime was deliberated, or what impact the knowledge had.
• Studies have not tried to discern if the threat of life without parole makes an equal impression on potential murderers.
• Violent crime rates have fallen nationwide due to an aging population, tougher incarceration of criminals and other factors. Pinning the death penalty's share of that decline is fruitless.
• Many states have death-penalty laws but rarely if ever get to an execution, muddying the potential impact on criminal minds. Colorado has executed one prisoner since 1967 — in 1997.
A flurry of pro-deterrence study results landed in the past decade, largely from economists and statisticians who said they could filter crime rates for dozens of factors and still get conclusive answers.
A 2002 study said each execution prevented five to six murders, and pardons from death row led to more murders.
Soon after, Emory University researchers, including Paul Rubin, said that national sorting of crime rates showed each execution prevented 18 deaths.
A colleague of Rubin's did a later analysis saying the effect did not hold true in states like Colorado that go decades between executions. Infrequent executions can actually increase the murder rate in those states through a "brutalization effect," said Emory law school researcher Joanna Shepherd.
However, if a state executes many people, then criminals become convinced the state is serious about the punishment, and the criminals start to reduce their criminal activity," Shepherd wrote.
"We don't look at the mind of the criminal," Rubin said. After allowing for differences between counties and states on race, poverty, age and other factors, the impact was real and measurable, he said.
The Emory study and others have looked at thousands of counties individually and calculated how soon after an execution the crime rate might have changed.
The main criticism of such studies is that executions are so rare, and skewed by just a few states like Texas conducting most of the executions, that the numbers are spread too thin to mean anything.
The New York Times pointed out that in 2003, with 16,000 homicides nationally, only 153 resulted in death sentences, and only 65 people were actually executed.
Two Pepperdine University professors made a quantitative analysis in 2007 that argued an even greater role for deterrence, saying each execution from 1979 to 2004 stopped 74 murders the next year.
To read more: No credible evidence on whether death penalty deters, experts say - The Denver Post http://www.denverpost.com/news/ci_23374844/no-credible-evidence-whether-death-penalty-deters-experts#ixzz2VR8hVDHN