It's said that correlation is not causation, but after they put a moratorium on the death penalty in 1967, the homicide rate skyrocketed. After they relented on the DP ten years later, the homicide rates started going down.
A number of skeptics of the efficacy of the DP said that it was mostly due from keeping violent offenders in prison longer. I'm for both...keeping them in prison and executing them when found guilty of murder beyond a shadow of a doubt. Hanging is my preferred method. No more needles.