Many wrongly believe that gross murder rates are the manner in which we detect deterrence. It isn't, nor can it be, even though some social scientists and others use that barometer.
For example, there are high, low and medium crime rates in different jurisdictions, throughout the world. Crime rates are constantly fluctuating through decades and centuries, throughout the world.
In all of those jurisdictions, and through all times, there will always be some who are deterred from entering criminal activity, based upon the fear of getting caught and the sanction to follow.
It is the same with the death penalty, as it is with all sanctions.
With the recent 25 USA studies finding for deterrence, they range in the deterrent effect preventing from about 90-900 murders per year, nationwide, or about 0.5%-5% of the total of all murders. For me, that is a huge number of lives saved, yet, it represents a very small fraction of the murder rate.
While no one can rationally or honestly say that the death penalty does not deter some, there will also never be any agreement on the measurement of the degree of that deterrence.
Some say that the burden of proof is with those supporting the deterrence hypothesis. Clearly, it is not. All prospects of a negative outcome deter some.
Obviously, the burden of proof is with those who say that the most sever sanction - execution - is the only negative outcome that deters none. Rationally, as with histories measure, it is a claim that cannot be defended.
Of course the death penalty deters.
The only questions, which will never be answered to anyones satifaction, is "How much does it deter?"
Based upon the recent studies, deterrence has very little effect, but that "little effect" represents saving 90-900 lives per year.
As Prof. Robert Belecker states:
"We support execution as a just and appropriate forfeiture of lives which deserve to be taken. We also support execution as a just and appropriate method to save lives which deserve to be saved. "
Please review:
Deterrence
All prospects of a negative outcome deter some. It is a truism. The death penalty, the most severe of criminal sanctions, is the least likely of all criminal sanctions to violate that truism.
25 recent studies finding for deterrence, Criminal Justice Legal Foundation,
http://www.cjlf.org/deathpenalty/DPDeterrence.htm "Deterrence and the Death Penalty: A Reply to Radelet and Lacock"
http://homicidesurvivors.com/2009/07/02/deterrence-and-the-death-penalty-a-reply-to-radelet-and-lacock.aspx "Death Penalty, Deterrence & Murder Rates: Let's be clear"
http://prodpinnc.blogspot.com/2009/03/death-penalty-deterrence-murder-rates.html "The Death Penalty: More Protection for Innocents"
http://homicidesurvivors.com/2009/07/05/the-death-penalty-more-protection-for-innocents.aspx