07 January 2019 :

Nations that abolish the death penalty then tend to see their murder rates decline, according to a December 2018 report by the Abdorrahman Boroumand Center, a Washington, DC-based organization that promotes human rights and democracy in Iran.
The report examined murder rates in 11 countries that have abolished capital punishment, finding that ten of those countries experienced a decline in murder rates in the decade following abolition. Countries were included if they met the following criteria: they had formally abolished the death penalty at least ten years ago, at least one death sentence had been imposed or carried out in the decade prior to abolition, and murder rate data was available from the World Trade Organization. The countries that met the study’s criteria were Azerbaijan, Bulgaria, Poland, Serbia, Estonia, Latvia, Ukraine, South Africa, Kyrgyzstan, Georgia, and Albania.
The researchers compared murder rates in the ten years after abolition of the death penalty to the baseline rate in the year of abolition. Six of the abolitionist countries (Azerbaijan, Bulgaria, Serbia, Estonia, South Africa, and Albania) experienced murder rates below the baseline all ten years following abolition. Four countries (Kyrgyzstan, Latvia, Poland and Ukraine) had either one or two years in which murder rates were higher than in the year of abolition, but saw murders fall below the baseline within five years and experienced overall downward trends. Only one country in the study, Georgia, saw murder rates trend upwards in the decade following abolition.
One decade after abolition, the murder rates in these countries declined by an average of six murders per 100,000 population.
The authors conclude, “Death penalty advocates’ fears that the state relinquishing the ultimate punishment will embolden potential criminals, or at least weaken deterrence, prove to be unfounded in light of this evidence.” The data is consistent with with state-level data in the United States, which has repeatedly shown lower murder rates in states that do not have the death penalty than in states that do and that the presence or absence of the death penalty does not appear to affect murder trends. A 2017 DPIC analysis found that abolishing the death penalty had no measurable effect on murder rates in general or the rate at which police officers are killed, contradicting popular arguments that the death penalty is necessary for public safety and to protect law enforcement officials.
The study is: What Happens to Murder Rates when the Death Penalty is Scrapped? A Look at Eleven Countries Might Surprise You. Abdorrahman Boroumand Center, December 2018.