Someone suggested that if IQ was measured with a mean of 1000 and standard deviation of 150, people might still be giving answers of about 1 to 5 IQ points (as in, answers that would translate to 0.1 to 0.5 IQ points the way we actually measure them).
But people have a clear analog for this: SAT scores. Someone willing to give up 5 points of IQ to smoke marijuana probably wouldn't balk at having to give up 20 points on the SAT.
An SAT is of limited use after you're admitted to college, though, so the question is plausibly not the same - I would be willing to trade hundreds of points on my SAT scores now, since I am not in high school - and if they're interpreting SAT scores to be the same, then you might as well have asked about IQ in the first place.
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.