Largest ever longitudinal twin study of adolescent cannabis use finds no relationship between heavy use and IQ decline.
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, particularly among adolescent users. Other analyses have found no or inconclusive evidence for a link between IQ decline and marijuana use. However, most prior research, regardless of conclusion, has suffered from small participant cohorts, absence of pre-use data, potential influence of confounding effects, or some combination of these factors.
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. Other tests, measuring abstract reasoning and problem solving associated with “fluid intelligence” showed no significant differences between marijuana users and non-users. 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.
The study also found no relationship between heavier or more frequent marijuana use and the magnitude of IQ decline. The authors note that their study, unlike some previous research, did not ask participants about their current use, but only the greatest use since the initial testing. So while the results cannot rule out a causal relationship between near-term use and cognitive ability, the findings do agree with previous studies finding no relationship between prior heavy marijuana use and long-term cognitive impairment.
The findings agree with previous studies finding no relationship between prior heavy marijuana use and long-term cognitive impairment.
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.
The authors conclude that taken together, the results provide “little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline”. While decline in some cognitive areas was detected, differences were also seen at baseline, no dose response was evident and relevant sibling pairs showed no significant changes. It’s possible that confounding factors and/or upstream causes of both marijuana use and cognitive decline are the true causative drivers of these behaviors.
Charlie Hatton | Meta Staff Writer