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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: 1, Insightful) by Anonymous Coward on Tuesday January 19 2016, @09:10PM
This assumes you know what factors to account for. Which you don't. Anyway it is on the people who claim such an effect to account for all these other factors, which they can never do. That is why this NHST driven research is of little value (not zero value because sometimes we learn something incidentally about data collection or whatever from it). If you have a theory there is an effect, you need to make a precise prediction about the numerical value of the effect or functional form of the dose response, etc. Something not vague like "decreases IQ", which is on level with astrology.
(Score: 2) by takyon on Tuesday January 19 2016, @11:45PM
You don't have to account for all of the factors. If you account for the factors you can think of, and the IQ decline/difference disappears, you have found the right factors.
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(Score: 0) by Anonymous Coward on Wednesday January 20 2016, @01:09AM
I'm not sure what significance IQ has in the first place, as it appears to be mere pseudoscience at best. That's what you get when you take the social 'sciences' seriously.
(Score: 0) by Anonymous Coward on Wednesday January 20 2016, @01:21AM
Can't factor A cancel out factor B? Instead I'd say account for everything you can think of and if the effect remains then you can attribute it to thc. But as said in the above post, there are just too many unless the effect is huge. Do what scientists always did before this NHST idea infested academia: Predict something precise.