Abolitionists will claim that most studies show that the death penalty has no effect on the murder rate at all. But that's only because those studies have been focused on inconsistent executions. Capital punishment, like all other applications, must be used consistently in order to be effective. However, the death penalty hasn't been used consistently in the USA for decades, so abolitionists have been able to establish the delusion that it doesn't deter at all to rationalize their fallacious arguments. But the evidence shows that whenever capital punishment is applied consistently or against a small murder rate it has always been followed by a decrease in murder. I have yet to see an example on how the death penalty has failed to reduce the murder rate under those conditions.
So capital punishment is very capable of deterring murder if we allow it to , but our legal system is so slow and inefficient, criminals are able to stay several steps ahead of us and gain leeway through our lenience. Several reforms must be made in our justice system so the death penalty can cause a positive effect.