and
Though the association was strong, the authors note that they can’t say for certain whether chronic and heavy cannabis use induces psychosis, or whether people prone to mental illness are more likely to be heavy users. It makes sense that people who feel the symptoms of incapacitating depression or mania, or who sense apparitions or voices only they can hear, might try to self-medicate with cannabis.
Having a psychiatric disorder diagnosed after someone is determined to be addicted to marijuana doesn’t mean the marijuana use caused it. The beginnings of the psychiatric disorder were likely there before the diagnosis of it, and those beginnings could have made marijuana quite attractive as self-medication. I also wonder if people who are connected enough to the mental health system to get an addiction diagnosis aren’t more likely to get diagnosed for psychiatric disorders they have.
How then, can we establish causality? In her article, Susan Pinker writes:
Without a randomized controlled trial, which would be unethical in the extreme, it’s hard to untangle these strands definitively.
I don’t think that is quite true. First of all, a randomized controlled trial wouldn’t be unethical if the intervention was something that lowered marijuana use rather than raised it. One simply needs to get a brilliant idea of how to get a randomly chosen half of one’s sample to reduce their marijuana use.
Also, in this case, different states legalizing marijuana at different times provides a reasonably good natural experiment for differences-in-differences estimation that assumes the trend lines in each state would have had an unchanging slope if it weren’t for marijuana legalization at different dates. Which states legalize isn’t random, but the date of legalization is still special.
