Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies

Abstract

Marijuana is one of the most commonly used drugs in the United States, and use during adolescence-when the brain is still developing-has been proposed as a cause of poorer neurocognitive outcome. Nonetheless, research on this topic is scarce and often shows conflicting results, with some studies showing detrimental effects of marijuana use on cognitive functioning and others showing no significant long-term effects. The purpose of the present study was to examine the associations of marijuana use with changes in intellectual performance in two longitudinal studies of adolescent twins (n = 789 and n = 2,277). We used a quasiexperimental approach to adjust for participants' family background characteristics and genetic propensities, helping us to assess the causal nature of any potential associations. Standardized measures of intelligence were administered at ages 9-12 y, before marijuana involvement, and again at ages 17-20 y. Marijuana use was self-reported at the time of each cognitive assessment as well as during the intervening period. Marijuana users had lower test scores relative to nonusers and showed a significant decline in crystallized intelligence between preadolescence and late adolescence. However, there was no evidence of a dose-response relationship between frequency of use and intelligence quotient (IQ) change. Furthermore, marijuana-using twins failed to show significantly greater IQ decline relative to their abstinent siblings. Evidence from these two samples suggests that observed declines in measured IQ may not be a direct result of marijuana exposure but rather attributable to familial factors that underlie both marijuana initiation and low intellectual attainment.M arijuana is one of the most commonly used drugs in the United States, with a lifetime use prevalence of 50% (1, 2). Despite this, research on the long-term effects of marijuana use is scarce compared with that of other illicit substances. Changing cultural attitudes toward marijuana have recently led to social and legal acceptance of recreational use (3, 4), making research on the potential consequences particularly salient.Previous research has shown that marijuana use can have a high societal cost through increased unemployment, absenteeism, decreased productivity, and increased rates of crime and incarceration (5-8). Given that about 19% of youth and young adults (ages 18-25 y) in the United States have used marijuana in the past month (9), the potential impact is nontrivial. There is evidence to suggest that the adolescent brain may be particularly vulnerable, especially with regard to neurocognitive functioning (10, 11). Marijuana use in adolescence, when the brain is still undergoing major developmental changes, has been associated with decreased intelligence (12, 13), reduced memory (13-15), poorer attention (16-18), and lower verbal ability (19-21). However, these findings come from cross-sectional studies, where the temporal ordering of cause and effect is uncertain. For longitudinal studies examining marijuana use and ch...

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“…However, this study was limited in terms of controlling for confounds and a relatively small number of the total study population in the affected group. Two recent large scale follow-up studies have found that cannabis use is not associated with reduced IQ when controlling for additional confounders and genetic factors 54,55 . These latter studies only followed participants up to the ages of 15 and 20 years and do not exclude the possibility that IQ might be affected if cannabis use and/or dependence is maintained for longer.…”

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“…These findings have been criticized as potentially representing a regression to the mean, given that the marijuana-using cohort had higher baseline IQ than the non-marijuana-using cohort. A recent longitudinal twin study found that IQ deficits observed in marijuana users may be attributable to confounding factors like familial and environmental influences rather than the direct neurotoxic effect of marijuana [55]. Of note, these studies examined a small number of IQ subtests and did not assess other measures of executive function or working memory that have been found to be affected by heavy marijuana use.…”

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