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A new study led by scientists from UCLA and the University of Minnesota attempts to examine the link between marijuana use and IQ in a larger context. Reporting in PNAS, the authors analyze results from two longitudinal studies of twins, with data from more than three thousand individuals from Southern California and Minnesota. Participants were tested in six cognitive areas at two time points: between 9-12 years of age, with follow-up between ages 17-20. Study subjects self-reported marijuana use and frequency, and additional information, including genetic data, family background, socioeconomic status and use of other substances was collected at each encounter.
The authors focused on three criteria they proposed as measures for evidence of a direct causal relationship between marijuana use and cognitive decline. First, 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. Second, with a causal link, a dose-response relationship – that is, higher decline with heavier marijuana use – would be expected. And finally, if the relationship is causal, then the association of marijuana use and IQ decline should remain, even after genetic and social factors are taken into consideration.
Among the six IQ-related areas tested, the authors found decreases in ability among marijuana users compared to non-users in two modules – Vocabulary and Information – associated with “crystallized intelligence”, or the ability to use learned knowledge.
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. The authors also focused on twins where one sibling used marijuana and the other didn’t, assuming similar genetic, socioeconomic and environmental factors for each member of the pair. These analyses, performed on more than 200 twin pairs, found no significant difference between users and non-users.
originally posted by: Phage
a reply to: Ghost147
You may be interested in this result:
Among the six IQ-related areas tested, the authors found decreases in ability among marijuana users compared to non-users in two modules – Vocabulary and Information – associated with “crystallized intelligence”, or the ability to use learned knowledge.
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The authors noted, however, that in one of the two studies, 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.
This studies exist and are used all the time in the fluoride debates.