If there is a mediating variable that captures all of the causal flow from "treatment" (smoking mrj) to "outcome" (iq), and moreover, this variable is not an effect of the unidentified common cause, you can use "the front door functional" (see Pearl's book) to get the causal effect.
If there is a variable that is a "strong cause" of the "treatment", but not of the "outcome" (except through treatment) then this variable is instrumental, and there are methods that will give you the causal effect using this variable.
If there is an observed effect of an unobserved common cause, and you know something about how this effect arose, there are methods for "reconstructing" the unobserved common cause, and then using the standard covariate adjustment formula.
In general (e.g. complex longitudinal cases), there is a neat algorithm due to Jin Tian that can handle all sorts of unobserved confounding. Not everything, of course. In general, unobserved confounders doom you.
If you want the full story, you can read for instance this paper:
http://ftp.cs.ucla.edu/pub/stat_ser/r336-published.pdf
There is also the issue of how to do this in practice with data and smart statistical methods, which is a long separate discussion.
Did they use this method in the statistical analysis of the study? It is behind a paywall for me.
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.