In Connecticut, Governor Jodi Rell vetoed a bill that would have banned capital punishment in the state. Her explanation couldn't have been any less intellectually or morally satisfying:
"There is no doubt that the death penalty is a deterrent to those who contemplate such monstrous acts. The statistics supporting this fact, however, are not easily tabulated."
The last I heard, if there's "no doubt" about something, it's possible to empirically verify it. At least, you know, when we're talking about public policy rather than the existence of God or the Easter Bunny.
Back here in the real world, the death penalty does not deter crime. It just doesn't.