Some more data to support the claims expressed in this post, and this one. There’s a paper here presenting the results of a survey among leading criminologists regarding their opinion on the deterrent effect of capital punishment in the U.S.
The findings demonstrate an overwhelming consensus among these criminologists that the empirical research conducted on the deterrence question strongly supports the conclusion that the death penalty does not add deterrent effects to those already achieved by long imprisonment.
Of course, it’s not because experts believe something that this corresponds to the truth, but at least it’s ammunition that can be used against those proponents of the death penalty who like to claim that there is a “scientific consensus” in favor of the deterrent effect. There is no such thing. On the contrary, if there’s a consensus, it’s for the opposing view.
Another point: this kind of statistic on expert opinion, together with the data offered in the posts I linked to above, is much more convincing than the data comparing murder rates in capital punishment states and abolitionist states.
At first sight, this graph also undermines the deterrent argument, but it’s not as solid as it appears. It’s always important to control your data for other variables which can explain a difference. Maybe there are other reasons why states without the death penalty have lower murder rates, e.g. less poverty, more gun control etc. And maybe the murder rate in states with capital punishment would be even higher without capital punishment.