Cannabis and schizophrenia: Scare stories are not policy arguments.

“Some” isn’t much of an argument. You need “How many?”

The Wall Street Journal has an op-ed by a Yale psychiatry resident warning that the legalization of cannabis will lead to increased disability due to schizophrenia. That’s based on a couple of studies showing that cannabis use, especially early heavy use, is a statistical risk factor for a subsequent schizophrenia diagnosis.

If I had a young friend with a family history of schizophrenia or who had experienced schizophrenic symptoms, I’d advise that person to stay away from cannabis. Why take unnecessary chances? But the evidence of an actual causal link is fairly underwhelming; it’s very hard to tell whether early cannabis use might reflect attempts at self-medication for pre-clinical symptoms rather than being an actual precipitating cause.

At the population level, we have what seems to me like strong negative evidence on the question whether increasing the availability of cannabis will lead to a measurable increase in the number of people with disabling levels of schizophrenia. Those in pre-Boomer and early Boomer birth cohorts in the U.S. – anyone born before about 1952 – had essentially zero experience with cannabis before the age of 18. But that changed rapidly. More than 10% of the high-school seniors of 1979 – roughly speaking, the birth cohort of 1961 – were daily or near-daily pot-smokers. Then the prevalence of heavy adolescent use fell sharply for a little more than a decade, reaching its trough around 1992, and has rebounded since.

Yet the rate of schizophrenia diagnosis shows no corresponding cohort-to-cohort swings. (Nor, for that matter, between high- and low-cannabis-prevalence areas within the U.S. or cross-nationally.) That doesn’t mean there aren’t individual cases in which cannabis precipitates a first psychotic break, or that there aren’t some people with schizophrenia whose disease course is worsened by pot-smoking. (It’s also possible, of course, that cannabis provides valuable symptomatic relief for others, but no doubt our vigilant Institutional Review Boards will protect us from ever learning about that phenomenon, if it exists.)

But those results do help put an upper bound on the number of additional schizophrenia diagnoses we need to fear as the result of the increase in cannabis use that would result from legalization and the resulting changes in availability, price, and attitudes. And that upper bound is fairly low: low enough to keep it off the list of the top ten reasons for or against legalization. Note, for example, that people put in jail or prison are at risk of severe damage, especially if while inside they become victims of physical or sexual assault. Some commit suicide. So the mental-health costs of arresting 650,000 people a year, and holding 30,000 or more prisoners at any one time, for cannabis offenses – costs that would be largely, though not entirely, abolished by legalization – might easily match or exceed the mental-health costs of increased exposure to cannabis.

The author of the WSJ piece solemnly announces, “The claim that marijuana is medically harmless is false.” No sh*t, Sherlock! Nothing is harmless. It’s always a question of counting harms, weighing them against one another, and comparing them to benefits. And we should do that not only when embarking on “social experiments” (i.e., making changes) but also when continuing a high-cost and potentially unsustainable status quo policy.

The costs of cannabis prohibition are large (including $35 billion a year in criminal income), and its capacity to keep consumption in check appears to be breaking down. That’s not a reason to plunge wildly into legalization on the libertarian model, but it is reason enough consider, soberly, the options around legal availability. Mere unquantified viewing-with-alarm (about schizophrenia, or workplace impairment, or intoxicated driving, or increased use by adolescents, or increased substance abuse disorder) no longer counts as a valuable contribution to the debate, any more than mindless sloganeering about “The failure of the War on Drugs.”

Some people will get hurt as a result of legalization; some people are getting hurt now by prohibition. The question before us is, “What policy would minimize total damage, net of the benefits of responsible use?” Continued prohibition in some form – at least the prohibition of commerce – might turn out to be the answer to that question; at least, Jonathan Caulkins and Keith Humphreys both think so, and they’re two of the most thoughtful and knowledgeable people around on this issue.

But being “against legalization” does not by itself name a policy position. No one I know has a serious proposal to put the genie back in the bottle, reversing the trend toward more cannabis use, and heavier use, that started around 2003 and seems to be accelerating. So it’s time to try some innovation. Who knows? We might be able to construct a licit market, and norms of responsile use, that would stop the progression toward more potent and less CBD-buffered, and thus probably riskier, cannabis. And then we should evaluate the results of those innovations with as much cool detachment as we can summon up: not to “prove” that one team of culture warriors or the other was right, but to consider what to try next. That’s the way grown-ups make policy.