By Jon Silverman
Legal Affairs analyst
As former Met Police Chief Lord Stevens calls for the death penalty to be reintroduced for police killers, BBC News examines the deterrent effect of punishment.
Pc Sharon Beshenivsky's murder prompted Lord Stevens' call
There has been relatively little research in the UK on the effect of punishment on homicide rates compared with other common law countries such as Canada and Australia.
And in the US, there is a plethora of evidence in the public domain, though there are big question marks over how rigorous much of it is.
Analysing the deterrent effect of the death penalty, Professor Jeffrey Fagan, of Columbia University, told a state commission in January 2005 that the studies "produce erratic and contradictory results".
But, in the main, he is critical of those who argue a causal relationship between capital punishment and falling murder rates.
One of the difficulties is in gauging the impact of other factors, such as length of sentence and clear-up rates, on the incidence of murder.
And there is no reliable evidence to show that murderers are aware of executions in their own state, much less in states further away.
Rising murder rate
Looking specifically at murders of police officers, a 1998 FBI report showed that in the south, the region where the death penalty was used most frequently, 292 law enforcement officers were killed in a ten-year period. Whereas, in the north-east, which had fewest executions, the figure was 80.
A 1995 poll of US police chiefs showed that at least three out of every five did not believe that the death penalty reduced significantly the number of homicides.
Indeed, as a factor, they rated it below drug abuse, the state of the economy and longer jail sentences.
However, researchers at Emory University in Georgia came to the conclusion in 2002 that every execution resulted in around 18 fewer serious crimes.
In England and Wales, the murder rate has risen substantially since the death penalty was abolished in 1965.
The population has gone up, is less homogenous and there are far more lethal weapons in circulation
But the population has gone up, is less homogenous and there are far more lethal weapons in circulation.
Rupa Reddy, a researcher at the University of Westminster's Centre for Capital Punishment Studies, said: "You find that most research in the States is being used to buttress a polemical argument, either for or against executions.
"You can't say that the death penalty has no deterrent effect. But there is no evidence that it is more influential than Life Without Parole sentences and first-class detection techniques."
Perhaps the most striking difference between Britain and the US is the number of homicides committed with firearms.
In the States, it is around 70%. Here, some 6%.