Cannabis et intelligence

Limitations.

This study is not without its limitations. Of primary concern is the inability to delineate precise mechanisms responsible for the association between marijuana use and IQ change. Many explanatory mechanisms may exist whereby marijuana use has an indirect effect on IQ. Marijuana is also associated with amotivational behavior, increased truancy, and lower educational attainment (47, 56), which are reflected in some IQ measures. Additionally, as Pope et al. (19) argued, marijuana users may be culturally divergent from the rest of society and consequently do not acquire the same skillsets that are measured on tests standardized to mainstream culture. The absence of evidence for a direct effect of marijuana use on IQ in the present study demonstrates the need for future research to focus on these alternate mechanisms.

Furthermore, the examination in this study was limited in scope in that we examined a small number of IQ subtests and did not examine measures of executive function or working memory. Differences between marijuana user groups have appeared in these other cognitive domains (18, 23, 57, 58). Nevertheless, there is no reason to expect such a difference would emerge in the present study. A subset of participants from the MTFS (n = 1,452), in fact, completed the WAIS-III (59) Digit Span subtest””a measure of working memory””during their follow-up assessment at ages 17–18 y. Those who used marijuana in adolescence performed slightly better on this test (mean, 9.4; SD, 2.4) relative to their abstinent peers (mean, 9.2; SD, 2.4). Despite these null findings, our results have no bearing on whether marijuana contributes causally to declines in other measures of cognition or other important outcomes such as psychosocial adjustment and mental and physical health.

Attrition is also a concern for this study, because there were a number of participants who were excluded for failing to have marijuana use information. It is likely that individuals with greater severity of marijuana use are less likely to participate in the later waves, potentially skewing our analyses toward null findings by examining only the least severe users. Despite this possibility, we note that ∼21% of our marijuana users were daily users for greater than 6 mo. Additionally, we found that those missing marijuana use information were significantly more likely to have lower IQ at baseline. There is some evidence to suggest that low IQ may pose a risk factor for marijuana use (46), which, if so, may further complicate the situation.

Our marijuana users were a heterogeneous group that included individuals with normal adolescent drug experimentation as well as more regular users. Although we examined dose–response relationships to isolate the effects of heavy use, we cannot rule out a neurotoxic effect of long-term marijuana use, particularly given that our measures were discretely measured and did not account for how recently the participants had used marijuana. Although our study evaluated a subset of subjects who had been daily users over the course of 6 mo to a year, all participants were still young adults at their last assessment. Perhaps regular use over a more prolonged period would result in greater differentiation across groups. This is a possible explanation for the discrepancy of our findings from those of Meier et al. (23), whose subjects were aged 38 y at their last assessment. We additionally were not able to differentiate current heavy users from former heavy users, and thus our results cannot disentangle the acute from nonacute effects of marijuana use on intellectual functioning.

Lastly, our measures of marijuana use relied upon participant recall, which may underestimate their substance use. This recall bias may be exacerbated in our frequency measures, particularly given the multiyear period between assessments. Our measure of binge drinking may be especially prone to memory errors, because it would be difficult for participants to recall the maximum number of drinks ever consumed in a day, particularly if they were intoxicated. Although our drinking measure should, in many cases, be viewed as an approximation rather than a precise count, we believe it adequately discriminates between abstinent/modest drinkers and heavy drinkers.

Conclusion

In the largest longitudinal examination of marijuana use and IQ change, using two samples to replicate results and a genetically informative design, we find little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline. Although marijuana users showed greater decline than nonusers in areas of Gc, the presence of baseline differences before marijuana involvement, the lack of a dose–response relationship, and an absence of meaningful differences between discordant siblings lead us to conclude that the deficits observed in marijuana users are attributable to confounding factors that influence both substance initiation and IQ rather than a neurotoxic effect of marijuana.

Le test de QI ne mesure pas ni ne prétend mesurer

Très juste.
Il s'agit d'évaluer des compétences, des performances sur un instant T. C'est surtout l'interprétation qui est à  remettre en cause.

Bien sûr que non, ce n'est qu'un aspect limité de l'ensemble des compétences humaines.

C'est toujours bon de le préciser. Malgré tout, cela peut être un bon outil dans certaines circonstances.

Dernière modification par Mister No (12 octobre 2016 à  11:14)