I don't believe the death penalty is a deterrent, despite studies that might suggest otherwise. Most people who commit murders either do not expect to be caught or do not carefully weigh the differences between a possible execution and life in prison before they act. Frequently, murders are committed in moments of passion or anger, or by criminals who are substance abusers and acted impulsively. Also, states without the death penalty consistently have lower murder rates than states with the death penalty.
I'm sure a review of the few studies saying that the death penalty is a deterrent would reveal significant flaws and inconsistencies in those studies.
- Assassination of a law enforcement officer, judge, juror or prosecutor for the purpose of obstructing an ongoing criminal proceeding
- Prolonged torture or multiple murders
- Defendant has already been convicted of first-degree murder or who is serving a life sentence without parole
Sounds like a surprisingly strong deterrence effect. If it's accurate, it's a pretty strong argument for the death penalty.