I wrote a piece for PajamasMedia on why I no longer support decriminalizing marijuana, because of the large number of studies demonstrating that marijuana use precedes schizophrenia, and the enormous social costs that this imposes. Not surprisingly, there are several groups commenting:
1. People arguing (sometimes quite calmly) that the social costs of prohibition are higher than the social costs of having it legal.
2. People arguing that this is all lies, marijuana isn't a problem at all! (Acolytes of the herb god, I think.)
3. People arguing that we should let mentally ill people die in the gutters if marijuana causes them mental illness problems. The libertarian ideal is so strong to such people that they do not realize that most Americans do not buy this argument.
4. People arguing that marijuana laws don't have any influence on behavior--no matter what the laws are, the same number of people will smoke pot. Yet, at the same time, they acknowledge that having it illegal drives up prices, attracting the violent criminals into the trade. Somehow, rising prices don't affect demand or consumption.
One of the strongest arguments for keeping marijuana illegal is to prevent it from getting such a strong toehold that, like alcohol, it becomes impossible to keep from becoming dominant, with the negative consequences of widespread use. It would appear that perhaps it is too late--that is already, like alcohol, a fundamental part of our culture.
UPDATE: By the way, this recent article in Archives of General Psychiatry, "Association Between Cannabis Use and Psychosis-Related Outcomes Using Sibling Pair Analysis in a Cohort of Young Adults," also concludes that marijuana use, especially prolonged use, increases the risk of psychosis:
Compared with those who had never used cannabis, young adults who had 6 or more years since first use of cannabis (ie, who commenced use when around 15 years or younger) were twice as likely to develop a nonaffective psychosis and were 4 times as likely to have high scores on the PDI. Further analyses demonstrated that these findings were not due to a small group of individuals with psychotic disorders nor to individuals who were acutely intoxicated with cannabis when completing the PDI.
While the study agrees that there are people with mental illness problems who use marijuana for that reason, it also points to a feedback loop:
The nature of the relationship between psychosis and cannabis use is by no means simple. In keeping with previous findings,33 we confirmed that those with early-onset hallucinations were more likely to have longer duration since first cannabis use and to use cannabis more frequently at the 21-year follow-up. This demonstrates the complexity of the relationship: those individuals who were vulnerable to psychosis (ie, those who had isolated psychotic symptoms) were more likely to commence cannabis use, which could then subsequently contribute to an increased risk of conversion to a nonaffective psychotic disorder.