I think studies have enduringly shown that indeed, the existence of capital punishment does not act as a deterrent in cases of violent crime. Yet it's still one of the main arguments of the for people, again and again, no sources cited and murmured again and again so as if it's said enough, people will believe it's true without checking.
Well I checked:
The death penalty is not a deterrent; those who are against the death penalty claim that recent studies in the US do not support the view that capital punishment acts as a deterrent. [12]. It is also argued that anyone who would be deterred by the death penalty would already have been deterred by life in prison, and people that are not deterred by that would not be stopped by any punishment. This argument is typically supported by claims that those states that have implemented the death penalty recently have not had a reduction of violent crime. A stronger variant of this argument suggests that criminals who believe they will face the death penalty are more likely to use violence or murder to avoid capture, and that therefore the death penalty might theoretically even increase the rate of violent crime. [13].
from the wikipedia.
Well I checked:
The death penalty is not a deterrent; those who are against the death penalty claim that recent studies in the US do not support the view that capital punishment acts as a deterrent. [12]. It is also argued that anyone who would be deterred by the death penalty would already have been deterred by life in prison, and people that are not deterred by that would not be stopped by any punishment. This argument is typically supported by claims that those states that have implemented the death penalty recently have not had a reduction of violent crime. A stronger variant of this argument suggests that criminals who believe they will face the death penalty are more likely to use violence or murder to avoid capture, and that therefore the death penalty might theoretically even increase the rate of violent crime. [13].
from the wikipedia.