To explore the question, they look at executions and homicides, by year and by state or county, trying to tease out the impact of the death penalty on homicides by accounting for other factors, such as unemployment data and per capita income, the probabilities of arrest and conviction, and more.
Among the conclusions:
• Each execution deters an average of 18 murders, according to a 2003 nationwide study by professors at Emory University. (Other studies have estimated the deterred murders per execution at three, five and 14).
• The Illinois moratorium on executions in 2000 led to 150 additional homicides over four years following, according to a 2006 study by professors at the University of Houston.
• Speeding up executions would strengthen the deterrent effect. For every 2.75 years cut from time spent on death row, one murder would be prevented, according to a 2004 study by an Emory University professor.
In 2005, there were 16,692 cases of murder and nonnegligent 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."
Death penalty supporters could have told these scientists all along that the death penalty deters murderers. Our evidence? Once caught, murderers seek to avoid the death penalty. The fact that so many murderers so often can, through plea bargains and through moratoriums on executions (or, as in San Francisco, moratoriums on seeking the death penalty even for cop-killers), affects the risk calculations of others contemplating acts of murder.
The sentence of death should be handed down much more often than it is, and death sentences should be carried out far more swiftly and reliably. Investing in a robust appeals process -- something that those sentenced to "execution by incarceration," life without possibility of parole, deserve no less than those sentenced to death -- would help shorten the time from conviction to execution, and save many innocent lives.