Academics from the University of Otago have torn apart claims made by Family First about the links between cannabis, psychosis and violence in a paper published this morning.
Prompted by a Family First petition calling for a government inquiry, the paper critiques a "tendentious" argument drawing on cherry-picked science which perpetuates the stigmatisation of people with serious mental illness.
Joseph Boden, co-author of the paper and an associate professor of psychological medicine at the University of Otago, said he believed Family First were deliberately misrepresenting the science on cannabis and contributing to a climate of misinformation ahead of next year's referendum.
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"It's the old fake news problem. If someone doesn't take the time to actually vet things, which most people don't, then what you're doing is you are creating a citizenry who are going to exercise their vote without all the facts, or with false facts, or with an alternative narrative that doesn't do justice to the subject at hand," he said.
A press release accompanying the petition claimed international studies have found THC is strongly linked to schizophrenia, psychosis and violence, but Boden said there were a number of issues with Family First's conclusions.
"They want to draw a straight line between cannabis and violence, and there's really no credible evidence of a straight link between the two.
They want to say it's going to cause psychosis and psychotic people are violent. Well, the first part is it's probably not a strong cause of a psychotic illness. There's very little evidence for that.
"The other thing is that they're trying to link psychosis to violence. There are psychotic people who are violent, but they're drawing from studies of hospitalised psychosis patients who have been sectioned against their will."
Family First were also misrepresenting the findings of the studies they reference, Boden said.
"It's very sloppy, but I think intentionally so because they already have an anti position and they're looking for stuff that they think can support it.
"As scientists, one of the things that we actually have to do apart from maintaining our objectivity about these things is actually to call out when people are misusing research."
Boden, a substance abuse specialist, is a member of the Christchurch Health and Development Study, which has followed a cohort of more than 1,200 subjects for 40 years, tracking their health, education and life.
He said the study has provided researchers with world-class data on the effects of cannabis use, and provides a strong platform to have an informed discussion in New Zealand.
"Our research on the psychosocial harms related to cannabis has been some of the best in the world, particularly the contributions of the Dunedin Study and the CHDS. I think we have a terrific basis to start this discussion in New Zealand.
"But I think that people who are engaging in a lot of non-fact based activity – and I'm also thinking of people like Mike Hosking – the spotlight that they take up is far too great."
Ahead of the referendum, the Prime Minister's chief science advisor will release a short, unbiased summary of the evidence for the harms and benefits of legalised cannabis, her office announced last month.
Professor Juliet Gerrard, with assistance from a panel of New Zealand researchers and academics, will draw from longitudinal studies like the CHDS and a wealth of international data from areas where cannabis has been legalised and decriminalised.
The report will be audited by international researchers and released to the public in January next year. Boden said it was an important resource to provide ahead of a vote.
"I think it's a great step and a necessary step to have all that expertise and building a fact-based document that will serve as a resource for citizens and for the government itself."
"It's not going to take a position for or against, but rather provide the best current evidence that we have around this."
"What's really important to note in that is that there hasn't been huge increases in use. Places like Portugal that decriminalised all drugs, use of drugs went down."
Last year, Family First accused an American academic of cherry-picking data when he pushed back on their claim that legalisation had led to a huge increase in cannabis use in Colorado.
"Since legalisation in our state, cannabis isn't used at higher rates than it was already before," he wrote. "But now, it's non-criminalised, regulated, and brings in tax revenue to our school districts, while not attracting drug cartels. You're welcome."
Family First called it fake news, but the state of Colorado said in a report there had been no increase in youth usage, and rates were the lowest in a decade.
But the accusation could be a sign of something more sinister than a few local advocates getting trigger-happy, Boden said.
"What worries me is that it looks to me like opposition is very well funded and coordinated, and why I say it that way is because I think most of us in the field probably agree that a legalised and highly regulated scheme is better than prohibition. It's a bad law and it's failed and it's generally applied in a racist manner."
"But if you look at the anti-cannabis lobby groups in the United States, as I have, it's a matter of public record who their funders are, and their funders are pharmaceutical companies."
"I think if the New Zealand public were to find out for example that major overseas international corporations were trying to influence their vote by infecting us with fake news, I think Kiwis would be pretty unhappy.
"Do I think it's likely that is happening? I don't doubt it for a second."
Update: In a statement to The Spinoff, Family First maintains there is a link between cannabis, psychosis and violence.
"In the same way that there is some real evidence that components of marijuana can be made into medicine, there is building scientific evidence suggesting that components of the plant can lead to mental illness, at times severe, that can lead to violence," they said.
"We are simply asking for research and scientific consensus before moving forward as a country with a change this massive."