When the New York Times Runs An Article Like This...

You know that the evidence is becoming overwhelming. From the November 18, 2007 New York Times:

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, 3 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 influential 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 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. “I did shift from being against the death penalty to thinking that if it has a significant deterrent effect it’s probably justified.”
Professor 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.
“Capital punishment may well save lives,” the two professors continued. “Those who object to capital punishment, and who do so in the name of protecting life, must come to terms with the possibility that the failure to inflict capital punishment will fail to protect life.”

As I have pointed out before, I am not comfortable with the death penalty. I'm not sure how certain we can be that it actually works as a deterrent. It is very expensive (at least partly because death penalty opponents have made it so). But it does seem that if it deters, then the occasional execution, done 15-20 years after the crime, is going to be far less effective than executions that are highly publicized, and happen often enough that a person considering murder will see them as at least a small possibility. One or two executions a year in an entire country are so remote a threat that a person considering murder has far more to be afraid of from his victim shooting back.