Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status (paywall link)
Does cannabis use have substantial and permanent effects on neuropsychological functioning? Renewed and intense attention to the issue has followed recent research on the Dunedin cohort, which found a positive association between, on the one hand, adolescent-onset cannabis use and dependence and, on the other hand, a decline in IQ from childhood to adulthood [Meier et al. (2012) Proc Natl Acad Sci USA 109(40):E2657–E2664]. The association is given a causal interpretation by the authors, but existing research suggests an alternative confounding model based on time-varying effects of socioeconomic status on IQ. A simulation of the confounding model reproduces the reported associations from the Dunedin cohort, suggesting that the causal effects estimated in Meier et al. are likely to be overestimates, and that the true effect could be zero. Further analyses of the Dunedin cohort are proposed to distinguish between the competing interpretations. Although it would be too strong to say that the results have been discredited, the methodology is flawed and the causal inference drawn from the results premature.
A week ago the meetup group in Berkeley discussed a new article in PNAS titled "Persistent cannabis users show neuropsychological decline from childhood to midlife". Several people who didn't attend said they were interested in how that conversation went.
Before discussing the specifics of the article, we went around the room and stated how many IQ points we'd be willing to spend for some level of cannabis use. The median answer given was 4 points for moderate usage. Then someone pointed out that since we responded out loud, there may have been an anchoring effect here.
My understanding of the group's understanding of the scientific result is that smoking so much marijuana that you're diagnosable as cannabis-dependent (whatever that means) before the age of 18 will give you an IQ hit of 9-11 points, maybe more, over 20 years, compared to nonusers.
People who were diagnosable as dependent on cannabis but not before 18 got an IQ hit of 4 or 5 points, on average. We don't know if this is because cannabis is bad for adults, or if it's bad for people just over 18.
People who have used cannabis but were not diagnosed as dependent got an IQ hit of 1 or 2 points on average. The article gives us little information on what the risks of various moderate levels of cannabis use are.
We didn't discuss any methodological errors in the study, but the general attitude of the group was that the scientific result is worth taking seriously.
After the discussion, people who use cannabis opportunistically or not at all — especially the younger attendees — said that after learning about the study they now have another reason not to use cannabis. One person who uses cannabis less than once per week said they wouldn't change their usage habits.