Research Professor Richie Poulton is the director of the Dunedin Multidisciplinary Health and Development Study, which has gathered information over the past 45 years about the lives of 1037 people born in 1972 and 1973.

Based on data from the Dunedin Study and the similar Christchurch Study, Mr Poulton has co-written a report called Patterns of recreational cannabis use in Aotearoa-New Zealand and their consequences: evidence to inform voters in the 2020 referendum.

In September, New Zealanders will vote on whether to endorse the Cannabis Legalisation and Control Bill, which would make cannabis possession legal for people aged 20 and older within their own homes and on licensed premises.

Mr Poulton told RNZ’s Saturday Mornings that 80 per cent of people in both the Dunedin Study and the similar Christchurch Study had tried cannabis at least once by the time they reached the age of 30.

“It’s entirely normative to have used cannabis, despite it being illegal throughout that time,” he said.

“The numbers suggest that the law is not penetrating or having the desired effect of deterrence.”

Study members who had been arrested or imprisoned for cannabis use did not tend to reduce their use afterwards – in fact, many increased their use, he said.

Those who used cannabis before the age of 18 had an increased risk of developing long-term psychosis or shorter term psychotic symptoms.

“There’s a whole bunch of changes to the brain architecture that go on during mid-adolescence in particular that make it far more sensitive to any substance,” Mr Poulton said.

“The evidence is there’s an approximate doubling of risk among people who start using cannabis during adolescence of developing psychosis in their mid-twenties.”

However, the risk of developing psychosis disappeared for those who tried cannabis when they were aged over 18, so Mr Poulton said the proposed legal age of 20 was appropriate.

The Dunedin study showed that about 25 per cent of the population had used cannabis by the age of 15 and about 25 per cent had a genetic predisposition to psychosis.

People with a vulnerable genotype “just need to stay away from the stuff – they’re not built to consume cannabis because they will have a much higher risk of becoming psychotic,” he said.

No matter how people chose to vote, the message needed to be clear that adolescents should avoid cannabis, Mr Poulton said.

The risks of lung damage and developing a dependence on cannabis are also higher for teenage users.

“You’re laying the foundations for a lifelong habit, because you’re starting at that key developmental point that could impact upon your cognition, your ability to finish school and graduate … and so forth.”

Mr Poulton said if cannabis was legalised, it would make it easier to teach high school students about the risks.

“You remove the stigmatising, quite frightening prospect from this scenario and all of a sudden it’s far more legitimate and effective to be talking about the risks of cannabis use, just like we educate about tobacco and alcohol misuse.”

People might also feel more comfortable seeking help if cannabis caused problems in their lives, he said.

“The current [legal] status acts as a barrier to people getting supports … and interventions that they need to manage the problems in the physical realm, in the psychological realm and in the social realm.”

The Cannabis Legalisation and Control Bill would set a tetrahydrocannabinol (THC) limit of 15 per cent.

Mr Poulton said cannabis potency had increased, so the study’s findings of the impacts of the drug would be “conservative”.

Only about nine per cent of people in the Dunedin Study were Māori and Mr Poulton said more focus on Māori and Pacific people was needed.

However, the studies had picked up a clear bias against Māori, who were more likely to be picked up by the legal system for cannabis offences.