Medical Marijuana: Data for Dummies

How many times do we have to go over correlation and causation?

Perhaps you've heard of Alex Berenson's book released last month, Tell Your Children: The Truth About Marijuana, Mental Illness, and Violence.

I first heard of the book in a tweet from, somewhat predictably, Arizonans for Responsible Drug Policy.

The link between cannabis and mental conditions such as schizophrenia, paranoia and psychosis has been the preferred trumpet of prohibitionists for a while now as they grasp for arguments to fuel their moral crusade.

Berenson was more than happy to help.

A credible journalist in his own right—former business investigative reporter for the New York Times covering the Bernie Madoff scandal and two tours covering Iraq—Berenson fell prey to a fallacy that arises all too often in data analysis: answering the question "what came first?"

But he's far from alone.

After hearing about an abundance of violent crimes associated with cannabis from his wife, a forensic psychiatrist, Berenson found similar suspicions from a 19th-century Indian lawyer and the Mexican government in the early 20th-century, according to Mother Jones.

Reviews describe Berenson's book as a collection of such observations of the link between mental illness and cannabis, citing the British Medical Journal and the National Academies of Science, Engineering, and Medicine.

Clearly, there is a correlation. That's difficult to refute at this point. But the argument for cannabis hasn't been that it's completely safe for a long time. It's safer than other drugs and it doesn't have a lethal dosage, so there's that. But many proponents now call for increased study of cannabis.

Unfortunately, the industry has far out-paced such studies and now we're flying with a foggy windshield.

But this link isn't new. It was described in a 2008 Australian academic study, which found "that cannabis use precipitates schizophrenia in persons who are vulnerable because of a personal or family history of schizophrenia."

If you read the first five words, you get the prohibitionist argument. If you read the last nine, you get the proponent argument.

A more recent 2014 study from Yale concluded "cannabis is neither necessary nor sufficient to cause a persistent psychotic disorder," while also determining more research is needed to fully uncover the link. A 2016 study from the Connecticut Veterans Affairs concurred.

The key here is that yes, there is a link. But what Berenson and the ARDP missed was what wasn't right in front of them. This is what happens when average people try to make claims from data they don't entirely understand.

It's unfortunate that Berenson's editors or someone at the ARDP didn't advise them to not overstate cannabis's effect on mental illness. Not only does it perpetuate false information, but they'd be far more credible pointing at the link instead of claiming to know everything about its complexity.

If you Google Berenson now, you'll find stories about his death threats and several articles (not a few of them from pro-cannabis publications) decrying his findings. Of course, death threats don't really help discourage his argument.

Perhaps if he'd tweaked his message, a lot of confusion and anger could have been avoided. Then again, it's probably good for sales.

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