Danish psychiatrist attacks Norwegian study linking cannabis and violence

May 13, 2014 - 04:55

Norwegian study claiming link between cannabis use and increased violence is heavily criticised by Danish psychiatrist.

Cannabis users are typically more violent than non-users but is cannabis the cause of the violence or is it something else? (Photo: Shutterstock)

A Danish psychiatrist has come out against a recent Norwegian study that claims to have found an association between cannabis use and violence.

Dr. Henrik Rindom of Hvidovre Hospital, Copenhagen, says the study is more political than based on actual sound science. Rindom has been counselling substance abusers for decades and studies the effect of intoxicants and narcotics.

The authors behind the Norwegian study says that a two-fold rise in cannabis use in Norway will lead to four per cent rise in violence -- but according to Rindom it isn’t so.

"I don't give much for this study," he says, adding that it seems like the studys’ authors are trying to get a political agenda across. “It [saying cannabis use increase violence] is like arguing that you become schizophrenic just by using cannabis.”

It’s a widely used statement without a shred of scientific evidence, says Rindom.

Read about the Norwegian study here on ScienceNordic: Association between cannabis use and violence

Violence caused by social traumas

However, Rindom does concede that cannabis users typically are more violent than non-users. But, and it’s an important but, he adds, the cannabis is not the cause of the violence -- rather it’s the violent behaviour that might lead people to use cannabis.

"Many young people use cannabis to subdue psychosocial traumas," says Rindom. "Cannabis doesn’t trigger their violent behaviour -- instead it’s their social problems.”

He says some people might use cannabis as an attempt at self-medication. And once they’ve become accustomed to the drug, they become even more violent if they don't get it, says Rindom.

To look at statistics only, like the Norwegian researchers did, makes no sense, says Rindom and emphasizes that cannabis is not the root of the problem.

"That’s not my experience of reality," says Rindom who counsels cannabis users -- and abusers. Take a look at young people in prisons, he says. Most of them consume cannabis on a regular basis and elicit violent behaviour at the same time. However, the reason for this is typically a difficult social history, says Rindom.

“Nobody deals with their fundamental problem: severe psychosocial traumas,” he says. “They should have psychotherapy instead of being left to medicate themselves."

Impulsiveness can lead to violence

The new study was carried out by researchers from the Norwegian Institute for Alcohol and Drug Research (SIRIUS). The researchers used statistical data on Norwegian youth from the years 1994-1999 to assess the association between cannabis use and violence.

Rindom says the researchers could use a check-up on reality.

"I lack an explanation as to why the link is there,” he says, asking if the violence could be a consequence of criminal activities that some abusers resort to as a mean to get their drug. “That could be an explanation."

"As my old tutor in medical statistics used to say: 'statistics are like old street lamps – they don't give much light, but they're good for supporting you during a storm'."

Study authors: we've taken uncertainties into account

The researchers from SIRIUS says they don’t understand Rindom’s criticism and Ingeborg Rossow, who co-authored the study, blankly denies that she or any of her colleagues are trying to be political.

Rossow says their scientific article makes certain reservations to the interpretation of their results.

For instance, that there might be certain factors they haven’t taken into account.

"We actually explain in the article why it’s reasonable to assume that a number of individual factors are connected with both the use of cannabis and aggressive behaviour,” she says.

That’s important to take that into consideration when interpreting the covariance between cannabis use and violence, she says. To get around that, the researchers used a type of statistical modelling that removes the importance of individual factors when they don’t vary over time, says Rossow.

This method is called fixed effects modelling and it actually makes it much harder to find a link between two thins -- such cannabis use and violence. “Nevertheless, we did find a small but statistically significant link," says Rossow.

The problem of correlation and causation

Rindom maintains that it’s problematic if researchers look only at the statistics. They’re too easily misunderstood, he says.

A classic example is the apparent connection between births in Denmark and storks. In the 1960’s Danish women started giving birth to fewer children and this coincided with a fall in the number of breeding storks in Denmark. One might jump to conclusions and claim that babies indeed are delivered by storks.

Of course, that wasn’t the case -- in reality both declines were caused by to separate incidents: the birth of the contraceptive pill and the disappearance of stork habitats.

"That's why it is so important to ask about the cause," says Rindom. "We can't do without statistics but we must be critical as statistics can be misunderstood and used politically."
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Read the full story in Danish on Videnskab.dk

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Translated by
Michael de Laine