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Meta at Science News reports on a new study (DOI: 10.1073/pnas.1516648113) still paywalled at PNAS:
Marijuana is used more than any other recreational drug, with recent trends toward greater social and legal acceptance in some regions. Concerns remain, however, about a possible causal relationship suggested in scientific studies between marijuana use and decline in IQ.
A new study from two longitudinal studies of twins, examine the link between marijuana use and IQ using data from more than three thousand individuals from Southern California and Minnesota.
The study by scientists from UCLA and the University of Minnesota focused on three criteria they proposed as measures for evidence of a direct causal relationship between marijuana use and cognitive decline.
- If marijuana use causes IQ decline, as opposed to merely being associated, then poor cognition scores should only be evident after use begins, and not before.
- If a causal link exists, a dose-response relationship would be expected.– that is, higher decline with heavier marijuana use.
- finally, if the relationship is causal, then the association of marijuana use and IQ decline should remain, even after accounting for genetic and social factors.
In tests of abstract reasoning and problem solving associated (called "fluid intelligence") showed no significant differences between uses and non users.
[more]
The study did find decreases in ability among marijuana users compared to non-users in the ability to use previously learned knowledge. (Vocabulary and Information retrieval, or so called "crystallized intelligence".)
The authors noted, however, that the baseline IQ scores of eventual users were already significantly lower in the affected areas.
Here, marijuana use does not precede cognitive decline, and they point out prior evidence that suggests other factors such as behavioral disinhibition and conduct disorder that may predispose individuals to both lower IQ and substance use.
(So criteria 1 above was not met).
The study also found no relationship between heavier or more frequent marijuana use and the magnitude of IQ decline.
(Criteria 2 was not met).
Finally, the authors examined the effects of outside factors associated with IQ decline. They found the decrease in Vocabulary scores was reduced in one study and "completely eliminated" in the other when adjusted for participants who self-reported binge drinking and use of other drugs.
(Criteria 3 also failed).
The authors conclude that taken together, the results provide "little evidence to suggest that adolescent marijuana use has any direct effect on intellectual decline".
(Score: 2) by ikanreed on Tuesday January 19 2016, @09:53PM
Well, it definitely does substantively affect likelihood of psychosis. Psychosis can be far more debilitating to classroom achievement than fluid intelligence.
Many people think of psychosis as a condition that results in severe delusions or hallucinations, but the more common form of it just makes it very hard to string cohesive thoughts together in order. That's even the main diagnostic criterion: how likely a subject's speech is to change subject mid-sentence. To your general "sense" of how smart someone is, psychosis can read very negatively, while not necessarily having any apparent impact on focused interpretation like IQ measures.
(Score: 1, Touché) by Anonymous Coward on Wednesday January 20 2016, @02:26AM
Well, it definitely does substantively affect likelihood of psychosis.
[Citation needed]
(Score: 2) by ikanreed on Wednesday January 20 2016, @05:17AM
Meta-analysis of existing research on the subject [procon.org]
(Score: 2) by mcgrew on Wednesday January 20 2016, @06:49PM
From your link:
The study did say there was an association between marijuana use and psychosis, but not that marijuana was the causative factor. I knew several crazy (as in later diagnosed as mentally ill) kids when I was a kid, and all of them wound up being stoners. I've known a lot of stoners, and the crazy ones were nuts before they took their first toke.
Carbon, The only element in the known universe to ever gain sentience
(Score: 2) by ikanreed on Wednesday January 20 2016, @07:04PM
That's a very generous reading of a summary that also includes language like this:
These longitudinal studies surveyed for self-reported cannabis use before psychosis onset and controlled for a variety of potential confounding factors (eg, other drug use and demographic, social, and psychological variables). Three meta-analyses of these and other studies concluded an increased risk of psychosis is associated with cannabis use, with an odds ratio of 1.4 to 2.9 (meaning the risk of developing psychosis with any history of cannabis use is up to 3-fold higher compared with those who did not use cannabis).
In addition, this association appears to be dose-related, with increasing amounts of cannabis use linked to greater risk—1 study found an odds ratio of 7 for psychosis among daily cannabis users.
As it goes on to note, that's not definitive proof of a causative function(which is hard to study for a controlled substance, which can't be administered in randomized trials), but if it doesn't make you go "That's a pretty substantial relationship", you're willfully glossing over the material to arrive at a predetermined answer.
(Score: 2) by mcgrew on Wednesday January 20 2016, @07:20PM
I'll admit I didn't read the whole study.
Carbon, The only element in the known universe to ever gain sentience
(Score: 0) by Anonymous Coward on Wednesday January 20 2016, @10:51PM
Same AC here. Now that wasn't so hard, was it?
(Score: 2) by ikanreed on Wednesday January 20 2016, @11:57PM
I never said it was hard, I just assumed the relationship was common knowledge. Dunning Kruger effect: you write your competencies and knowledge onto other people around you.
(Score: 0) by Anonymous Coward on Thursday January 21 2016, @04:52AM
I never said it was hard, I just assumed the relationship was common knowledge. Dunning Kruger effect: you write your competencies and knowledge onto other people around you.
Not a good assumption.
I mean, what with smoking up all the time, then freaking out and beating up homeless guys, busting plate glass windows or playing mailbox baseball, who can remember all the scholarly papers we read anyway?