A Dunedin study has traced the high level of violence among cannabis-dependent people to their history of anti-social behaviour and the illegal drug economy, rather than use of the drug itself.
A report released in London last week showed that cannabis-dependent 21-year-olds were nearly four times as likely to be violent as their peers.
It also found that those who were alcohol-dependent were nearly twice as likely to be violent than other 21-year-olds and those suffering from schizophrenia were 2 1/2 times more likely to be violent.
The study of 961 people born in Dunedin, carried out by the Dunedin Multidisciplinary Health and Development Research Unit, showed no relationship between occasional users of cannabis and violence.
Unit director Dr Richie Poulton said the problem was with the "small subset" who used lots of cannabis and were deemed to be dependent on the drug.
Dr Poulton said the Dunedin people were studied as 21-year-olds for violence, because that age was the peak for violent offending and mental health problems.
The study found people diagnosed as dependent on cannabis "had a long history of anti-social behaviour, going right back to when they were three years old ...
"They were being naughty, beating up other kids in the sandpit, being disruptive, then they went to stealing milk money, then they went to beating up bigger kids in the schoolground, then they converted a car ... It goes on and on and on."
The study found that most cannabis-dependent people were also drug dealers and involved in the black economy.
"When stuff doesn't work out right they just resort to violence," Dr Poulton said.
A report by an international group of psychiatrists on the Dunedin study noted that: "Among marijuana-dependent individuals violence was not explained by substance use prior to the offence, but was best explained by a juvenile history of delinquent behaviour, implicating involvement in illicit drug economies which resort to violence for dispute resolution."