By Max Daly
In the wake of the high profile conviction of cannabis smoker Thomas Palmer for the murder of his two friends, what is the truth behind marijuana's links with violence?
Cannabis has been downgraded to a category C drug
Reefer Madness, an anti-marijuana propaganda film shown in the US in 1936, begins with a teacher warning pupils and their parents against the dangers of marijuana.
The action follows his story of a group of students whose lives swiftly descend into mayhem and murder after they smoke 'reefers' and listen to jazz.
By the end of the cautionary tale, Jimmy has run down and killed a pedestrian, Jack has shot Mary, Ralph has gone insane and beaten Jack to death and Blanche has killed herself.
The documentary-style film was released a year before new anti-cannabis laws were introduced by Commissioner of Narcotics Harry J Anslinger, who told Congress: "If the hideous monster Frankenstein came face to face with the monster marijuana he would drop dead of fright."
The grim warning of Reefer Madness appeared to be played out for real this week with the jailing of teenager Thomas Palmer, who stabbed his two friends to death after a history of heavy cannabis use.
During his trial, a doctor who had been treating Palmer since his arrest told the jury: "I believe that his state of mind at the time of the killings was not normal.
"This was exacerbated, but not caused, by cannabis."
The Daily Mail instantly branded Palmer the 'drug-crazed killer' and the case sparked calls from senior police, politicians and newspapers to reverse the 2004 reclassification of cannabis from a class B to a class C drug.
The Liverpool Echo declared it had unearthed super-strength cannabis "so incredibly strong it can bring on the early signs of schizophrenia from a single puff."
Headlines linking cannabis with acts of violence are nothing new.
In the run up to the reclassification, Daily Mail readers were told that cannabis was to blame for:
- A vampire fantasist who drank the blood and ate the heart of his 90-year-old victim.
- A father smothering his baby son
- A teenager slaughtering a motorist while dressed as a samurai sword-wielding ninja
- A daughter bludgeoning her father to death with a poker
- Two crack addicted teenagers punching to death a baby and a mental patient strangling his former lover
These kind of headlines are not just pulled out of the ether in support of an editorial line.
Thomas Palmer was convicted of a double murder
They are gleaned from defence barristers who use the fact their client's judgement was clouded by smoking cannabis to plead diminished responsibility, judges who point the finger at cannabis use as a way of explaining motiveless crimes and coroners who choose to highlight an offender's marijuana intoxication over alcohol abuse or mental disorders.
In the case of Thomas Palmer, the jury rejected his plea of diminished responsibility - a state of mind which the defence sought to blame on cannabis.
That cannabis can cause mental health problems among heavy users is well documented and undisputed.
But there is little evidence to back claims that the drug, or its contribution to a decline in people's mental health, can itself trigger violence.
Virtually every piece of research carried out in the last 45 years debunks the cannabis causes violence myth.
No violence link
Psychoactive Substances and Violence, a 1994 report conducted by the US Justice Information Center, concluded: "Of all psychoactive substances alcohol is the only one whose consumption has been shown to commonly increase aggression.
"After large doses of amphetamines, cocaine, LDS and PCP certain individuals may experience violent outbursts, probably because of pre-existing psychosis."
The report does not even bother to include cannabis in the list of potentially violence-inducing substances.
The US National Commission on Marijuana and Drug Abuse said in its 1973 report, Marijuana: A Signal of Misunderstanding: "In sum, the weight of the evidence is that marijuana does not cause violent or aggressive behaviour.
This view is supported by the White House Conference on Narcotics 1962 and the President's Commission on Law Enforcement 1967.
Two of the most recent studies into the links between cannabis, mental health and violence, carried out among 1,000 young adults in New Zealand and published in 2000 and 2002, have been used as major planks in the cannabis causes aggression argument.
Yet Dr Louise Arseneault, the lead author on both studies, says her work has been misrepresented.
"We found that people dependent on cannabis were more likely to commit violent crime. But to say our studies showed that cannabis itself caused violence is wrong.
"We found it was not the substance that caused the violence, it was because heavy users were more likely to have a history of anti-social behaviour, bad parenting, failure at school, thieving and involvement in the illegal drug market.
"It is not because of consumption, it's because of past history.
"To say the paranoia created by smoking cannabis makes you more likely to be violent is a very big claim," she says, "there is no evidence for this."
While Harry J Anslinger's "monster marijuana" can certainly be blamed for a deterioration in the mental health in some users, its power as the driving force behind an endless string of murders and suicides is at present the stuff of fiction.