Take a look at this article which summarizes research supporting the deterrent effect of capital punishment. The article reports that each execution deters up to eighteen (18) additional murders. It turns out that the American public knew more about capital punishment's effect on the murder rate than the so-called opinion leaders did.
Predicting human behavior relies on history. Or as an old racetrack handicapper said, "Bet 'em the way they ran in the past." Not all of us are capable of killing someone on a sunny afternoon just because we're angry or greedy. If you've murdered in the past, you are capable of murder.
I'm of the opinion that even if the only murderer deterred by capital punishment is the executed party himself, capital punishment is still worth doing. A convicted murderer, once executed, will murder no additional folks.
