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Proc Natl Acad Sci U S A. 2016 Feb 2;113(5):E500-8. doi: 10.1073/pnas.1516648113. Epub 2016 Jan 19.

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

Author information

1
Department of Psychology, University of Southern California, Los Angeles, CA 90089; Department of Medicine Statistics Core, University of California, Los Angeles, CA 90024;
2
Department of Psychology, University of Minnesota, Minneapolis, MN 55455; jdisen@umn.edu.
3
Department of Psychology, University of Southern California, Los Angeles, CA 90089;
4
Department of Psychology, University of Minnesota, Minneapolis, MN 55455;
5
Department of Psychology, University of Southern California, Los Angeles, CA 90089; School of Law, Psychology and Social Work, Örebro University, 702 81 Örebro, Sweden;
6
Department of Criminology, University of Pennsylvania, Philadelphia, PA 19104; Department of Psychiatry, University of Pennsylvania, Philadelphia, PA 19104; Department of Psychology, University of Pennsylvania, Philadelphia, PA 19104.

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.

KEYWORDS:

adolescence; intelligence; longitudinal; marijuana use; twins

PMID:
26787878
PMCID:
PMC4747759
DOI:
10.1073/pnas.1516648113
[Indexed for MEDLINE]
Free PMC Article

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