The deterrent value of the death penalty has come into focus with a controversial study:

Anti-death penalty forces have gained momentum in the past few years, with a moratorium in Illinois, court disputes over lethal injection in more than a half-dozen states and progress toward outright abolishment in New Jersey.

The steady drumbeat of DNA exonerations _ pointing out flaws in the justice system _ has weighed against capital punishment. The moral opposition is loud, too, echoed in Europe and the rest of the industrialized world, where all but a few countries banned executions years ago.

What gets little notice, however, is a series of academic studies over the last half-dozen years that claim to settle a once hotly debated argument _ whether the death penalty acts as a deterrent to murder. The analyses say yes. They count between three and 18 lives that would be saved by the execution of each convicted killer.

The reports have horrified death penalty opponents and several scientists, who vigorously question the data and its implications.

The authors of the study defend the conclusions:

"Science does really draw a conclusion. It did. There is no question about it," said Naci Mocan, an economics professor at the University of Colorado at Denver. "The conclusion is there is a deterrent effect."

A 2003 study he co-authored, and a 2006 study that re-examined the data, found that each execution results in five fewer homicides, and commuting a death sentence means five more homicides. "The results are robust, they don't really go away," he said. "I oppose the death penalty. But my results show that the death penalty (deters) -- what am I going to do, hide them?"

Statistical studies like his are among a dozen papers since 2001 that capital punishment has deterrent effects. They all explore the same basic theory _ if the cost of something (be it the purchase of an apple or the act of killing someone) becomes too high, people will change their behavior (forego apples or shy from murder).

Those conclusions are startling:

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.

But what of the mistakes? Two infamous wrongful convictions in Canada come to mind -- Donald Marshall and David Milgaard.

Both men are now free, with the real killers behind bars.

So do we veer away from capital punishment because of what could have befallen Marshall and Milgaard? It is a compelling argument against capital punishment. But here is another compelling argument. Could capital punishment have save these two from years of wrongful incarceration? Think about it. If the death penalty does deter murder, would the victims at the centre of the Marshall and Milgaard trials (Sandy Seale and Gail Miller respectively) be alive today, their own murders having been detered? Would Roy Ebsary and Larry Fisher have not commited the crimes for which Marshall and Milgaard were convicted?

It's an unfair question, in a way, since it is impossible to answer one way or the other. Here's another unfair question. Even if the murders would have occurred with the death penalty was in place, would the subsequent miscarriages of justice that put Marshall and Milgaard behind bars been avoided? With the death penalty on the table, would more a rigorous system of checks and balances, and the promise of an automatic appeal, prevented the mistakes from happening, or caught them before Marshall and Milgaard spent decades incarcerated? Indeed, Marshall was committing a crime with Sandy Seale, intending to rob Roy Ebsary. Ebsary killed Seale and was ultimately convicted, but at the time, Marshall was evasive with police with regards to his actions that night. If the death penalty was on the table, would Marshall have been more honest with what had happened that night (the cost of admitting to the crime of attempted robbery being so much less than the cost of a murder conviction when the death penalty is an option)? Would that have pulled attention away from Marshall, or otherwise have altered the sequence of events during the investigation and trial? Would the checks and balances forced investigators to uncover the real sequence of events, even if Marshall insisted on being untruthful?

All unfair questions, but this study strongly suggests that the questions can't be dismissed out of hand, even if they can't be answered for any specific case.