tl;dr: There is no evidence supporting the conclusion that capital punishment is an effective deterrent for murder, and significant evidence to the contrary.
The only way to arrive at an alternative conclusion would be to ignore or refute:
- the results of multiple surveys done by the United Nations;
- a systematic review and rebuttal of all significant papers that ostensibly demonstrated statistically significant deterrence from capital punishment, by a Professor of Mathematics at Dartmouth College;
- a systematic review and rebuttal of all significant papers that ostensibly demonstrated statistically significant deterrence from capital punishment, by a Professor of Law & Public Health at Columbia University;
- the opinion of Justice Marshall of the Supreme Court of the United States who reviewed a "massive amount of evidence";
- the opinion of an overwhelming majority of leading criminologists in the USA; and
- the statistics in Canada that show a highly significant and consistent decrease in murder since the abolishment of capital punishment in 1976 (i.e. from a murder rate of 3.09 per 100,000 people in 1975, to 1.77 per 100,000 in 2009).
Amnesty International, who has an incentive in corralling information on this topic, provides an article, citing statistics from the United Nations, entitled “THE DEATH PENALTY, QUESTIONS AND ANSWERS” which states (with my emphasis):
What do you say to the argument that the death penalty is an important tool for a state to fight crime?
Too many governments believe that they can solve urgent social or political problems by executing a few or even hundreds of their prisoners. Too many citizens in too many countries are still unaware that the death penalty offers society not further protection but further brutalization.
Scientific studies have consistently failed to find convincing evidence that the death penalty deters crime more effectively than other punishments. The most recent survey of research findings on the relation between the death penalty and homicide rates, conducted for the United Nations in 1988 and updated in 1996 and 2002, concluded: "...research has failed to provide scientific proof that executions have a greater deterrent effect than life imprisonment. Such proof is unlikely to be forthcoming. The evidence as a whole still gives no positive support to the deterrent hypothesis".
Recent crime figures from abolitionist countries fail to show that abolition has harmful effects. In Canada, for example, the homicide rate per 100,000 population fell from a peak of 3.09 in 1975, the year before the abolition of the death penalty for murder, to 2.41 in 1980, and since then it has declined further. In 2003, 27 years after abolition, the homicide rate was 1.73 per 100,000 population, 44 per cent lower than in 1975 and the lowest rate in three decades. Although this increased to 2.0 in 2005, it remains over one-third lower than when the death penalty was abolished.
It is incorrect to assume that people who commit such serious crimes as murder do so after rationally calculating the consequences. Often murders are committed in moments when emotion overcomes reason or under the influence of drugs or alcohol. Some people who commit violent crime are highly unstable or mentally ill. Amnesty International has found that at least one in 10 of the prisoners put to death in the USA since 1977 had suffered from serious mental disorders rendering them unable to rationally comprehend their death sentence, its reasons or its implications.In none of these cases can the fear of the death penalty be expected to deter. Moreover, those who do commit premeditated serious crimes may decide to proceed despite the risks in the belief that they will not be caught. The key to deterrence in such cases is to increase the likelihood of detection, arrest and conviction.
The fact that no clear evidence exists to show that the death penalty has a unique deterrent effect points to the futility and danger of relying on the deterrence hypothesis as a basis for public policy on the death penalty. The death penalty is a harsh punishment, but it is not harsh on crime.
and, as capital punishment relates to terrorism and political violence:
Isn't the death penalty needed to stop acts of terrorism and political violence?
Officials responsible for fighting terrorism and political crimes have repeatedly pointed out that executions are as likely to increase such acts as they are to stop them. Executions can create martyrs whose memory becomes a rallying point for their organizations. For men and women prepared to sacrifice their lives for their beliefs -- for example suicide bombers -- the prospect of execution is unlikely to deter and may even act as an incentive.
State use of the death penalty has also been used by armed opposition groups as a justification for reprisals, thereby continuing the cycle of violence.
As an organization that seeks to protect human rights, the argument could be made that Amnesty International has a vested interest in this conclusion. However, I would expect that persuasive research to the contrary – i.e. that homicide rates decline in areas with capital punishment – would be referenced, by AI as a matter of protocol and integrity. I have seen no such references, by AI or otherwise.
The Canadian statistics referred to in the above can be found in a report by Amnesty International entitled “The Death Penalty in Canada: Twenty Years of Abolition”, which provides:
Contrary to predictions by death penalty supporters, the homicide rate in Canada did not increase after abolition in 1976. In fact, the Canadian murder rate declined slightly the following year (from 2.8 per 100,000 to 2.7). Over the next 20 years the homicide rate fluctuated (between 2.2 and 2.8 per 100,000), but the general trend was clearly downwards. It reached a 30-year low in 1995 (1.98) -- the fourth consecutive year-to-year decrease and a full one-third lower than in the year before abolition. In 1998, the homicide rate dipped below 1.9 per 100,000, the lowest rate since the 1960s.
Although the research in Canada only shows a correlation (i.e. not a causation), it does provide the specific statistics on a jurisdiction that abolished capital punishment – which was posed as part of this question. It is noteworthy that while Canada has uniform criminal laws across the country, its social policies are diverse as between its provinces, and yet the abolition of capital punishment lowered murder rates across the country.
To assuage any concerns that Amnesty International could be perceived to be biased, here's another reference, from John Lamperti, a professor of mathematics at Darthmouth College, entitled “Does Capital Punishment Deter Murder? A brief look at the evidence”, which seems to review most of the knowledge on the topic in the United States, and concludes:
Those who defend the deterrent value of the death penalty offer little
systematic research to support their view. Instead, they rely on an intuitive
feeling that capital punishment should be uniquely effective. When the
available evidence doesn't support that conclusion, they argue that the
evidence is imperfect. It is. But if there were any substantial net deterrent
effect from capital punishment under modern U.S. conditions, the studies we
have surveyed should clearly reveal it. They do not.
This view is affirmed by the systematic review by Jeffrey A. Fagan, Professor of Law & Public Health at Columbia University in his article “Capital Punishment: Deterrent Effects & Capital Costs”. In this article, Professor Fagan provides a thorough and highly critical analysis of the flaws in the statistical methods and conclusions of those who argue capital punishment is an effective deterrent:
When we apply contemporary social science standards, the new deterrence studies fall well short of this high scientific bar. Most of the studies fail to account for incarceration rates or life sentences, factors that may drive down crime rates via deterrence or incapacitation; one study that does so finds no effects of execution and a significant effect of prison conditions on crime rates. Another report shows incarceration effects that dwarf the deterrent effects of execution. Most fail to account for complex social factors such as drug epidemics that are reliable predictors of fluctuations in the murder rate over time.
...
The computations in the statistical models are often flawed. For example, simple corrections for large amounts of missing data produce estimates of the deterrent effect of execution that are no different from chance. Using alternate statistical models - models that account for the strong statistical correlation of murder rates from one year to the next - also produces results that show that changes in homicide rates are statistically unrelated to any measure of capital punishment.
Professor Fagan also notes that the rarity of capital punishment limits the effectiveness of its deterrence – it is applied so infrequently that it is incoherent to believe it would enter into the state of mind of potential murderers. Professor Fagan goes on to comment that no studies provide any evidence whatsoever that capital punishment ever enters into the state of mind of those who are about to commit murder (i.e. none of the studies even address causation):
... There is no evidence that if aware of the possibility of execution, a potential murderer would rationally decide to forego homicide and use less lethal forms of violence. Murder is a complex and multiply determined phenomenon, with cyclical patterns for distinct periods of more than 40 years of increase and decline that are not unlike epidemics of contagious diseases. There is nothing in the new deterrence studies that fits their story into this complex causal framework.
Much of the discussion in the USA goes back to a decision, Furman v. Georgia, 1972, of the Supreme Court of the United States, which decision briefly banned capital punishment in the United States. Professor Lamperti's paper opens with a quote from a decision of Justice Marshall, who was a Judge on that decision:
In light of the massive amount of evidence before us, I see no alternative
but to conclude that capital punishment cannot be justified on the basis of its
deterrent effect. — Justice Marshall, U.S. Supreme Court, Furman v. Georgia, 1972
Professor Lamperti's paper contrasts the quote of Justice Marshall with the views of President Richard Nixon, who expressed the opinion that “the death penalty can be an effective deterrent of specific crimes”.
It is worthwhile to note that the counterargument to Justice Marshall's conclusion was not that capital punishment was justifiable on the basis that there was evidence of it being effective. Rather, the other position taken before the Court seemed to be that prohibiting states from employing capital punishment was outside the scope of the powers of the US federal government. Unless I misread something in the reasons of the Court, none of the Judges of the Supreme Court stated that there was any evidence before the Court that capital punishment was effective.
Professor Lamperti notes that his view is consistently affirmed by leading criminologists:
Marshall's view is today supported by an overwhelming majority among America's leading
criminologists, who believe that capital punishment does not contribute to
lower rates of homicide [footnote: Michael Radelet and Ronald Akers, "Deterrence and the death penalty: the views of the experts," Journal of Criminal Law & Criminology 87, no.1 (1996), pp. 1-16.]
The above is affirmed in the more recent article “Do Executions Lower Homicide Rates? The Views of Leading Criminologists,” Journal of Criminal Law and Criminology 99 (2009): 489-508 (Michael L. Radelet and Traci L. Lacock). Few are as eminent in criminology as Professor Radelet, from whom I select a choice quote “The more people know about the death penalty the more likely they are to oppose it”.
Which is all to say: Capital punishment is not an effective deterrent for murder. Please permit me to finish with one final quote from Professor Lamperti:
If executions protected innocent lives through deterrence, that would weigh
in the balance against capital punishment's heavy social costs. But despite
years of trying, this benefit has not been shown to exist; the only proven
effects of capital punishment are its liabilities.
