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 uncontroverted 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, an organization with an incentive to corral 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 and is philanthropically funded as such, Amnesty International has a perceived bias towards this conclusion. However, I would expect that any 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.
Statistics since Canada abolished capital punishment
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.
Professor Lamperti, professor of Mathematics at Dartmouth College
Here is 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.
Professor Fagan, professor of Law & Public Health at Columbia University
The view of Professor Lamperti is affirmed by the systematic review of 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 study has ever been put forward that provides 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.
Furman v Georgia, 1972 (SCOTUS), opinion of Justice Marshall
Much discussion of this topic in the USA goes back to a decision, Furman v. Georgia, 1972, of the Supreme Court of the United States. Professor Lamperti's paper opens with a quote from a decision of Justice Marshall, 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 Justice Marshall's conclusion was uncontroverted. No Judge concluded 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 a jurisdictional issue i.e. the power to prohibit capital punishment was outside the scope of the powers of the US federal government. As I read it, none of the Judges of the Supreme Court stated that there was any evidence before the Court that capital punishment was effective.
Almost all Criminologists
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”.
The United Nations, Norval Morris and Marc Ancel
Criminologist Norval Morris prepared for the United Nations (and, incidentally, cited by Justice Marshall in the Furman case), citation Capital Punishment, UN Doc. ST/SOA/SD/9 UN Doc. ST/SOA/SD/10 (1968), Vol. II, p.123 states:
It is generally agreed between the retentionists and abolitionists, whatever their opinions about the validity of comparative studies of deterrence, that the data which now exist show no correlation between the existence of capital punishment the lower rates of capital crime.
This was one of several reports prepared for the United Nations from 1959-1980. See e.g. The Abolition of the Death Penalty in International Law, By William A. Schabas at pp.156ff. One notable report was by the eminent French jurist Marc Ancel, who concluded "all the information available appears to confirm that such a removal [of the death penalty] has, in fact, never been followed by a notable rise in the incidence of the crime no longer punishable with death."
It is worth noting that when these studies began in the 1950's the majority of UN state members were "retentionist" (i.e. they retained capital punishment, as opposed to abolitionist). In 1977, only 16 or so countries were considered abolitionist (not having a penalty of death), whereas by 2012 that number was around 97. In other words, the studies were commissioned and paid for by the United Nations at a time when the organization was predominantly composed of states that employed capital punishment.
State-sponsored killing of people in the name of capital punishment is ostensibly justified because it prevents further crimes. This deterrence falls into two categories, specific deterrence (i.e. recidivism by the convicted) and general deterrence (i.e. dissuasion of potential murderers other than the convicted).
As a specific deterrent capital punishment has no known benefits over life imprisonment. As a general deterrent, there has never been a study indicating that the state of mind of any murderer has ever been influenced by the possibility of capital punishment. Further, no jurisdiction has ever been mentioned for having a decrease in crime rates because it implemented capital punishment.
A persuasive counter-argument by an advocate of capital punishment ought to explain the dramatic drop in murder rates after Canada abolished it, the opinion of the majority of US criminologists and Justice Marshall, and the comprehensive academic and UN reports indicating an absence of deterrence from capital punishment. No such counter-argument has been forthcoming.
Which is all to say that capital punishment is not known to be an effective deterrent for murder. Please permit 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.