I interpreted the point AlexMennen raised as "people like dealing with small integers, and so may be giving poor answers because they don't have the right context." My response was that there is a larger scale measure of intelligence that Americans have meaningful context with. That context is both where their scores / their friends' scores are and what life outcomes are impacted by those scores. For example, the 25th percentile SAT reading scores for Stanford / UC Berkeley / UCLA / Sac State are 670/600/570/410, and so one could interpret a 30 point drop on each SAT test as about the difference between being a C student at Berkeley and a C student at UCLA. 11 IQ points is about the difference between being a C student at Stanford and a C student at UCLA (but IQ-SAT score conversions are wonky now that they clipped the right side off of the SAT distribution).
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.