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Study Questions Role for Marijuana in Teen Users’ IQ Decline

August 01, 2016
By Sheri Grabus, Ph.D., NIDA Notes Contributing Writer
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In a recent study sponsored by NIDA and the National Institute of Mental Health, teens who used marijuana lost IQ points relative to their nonusing peers. However, the drug appeared not to be the culprit. The new findings contribute to an ongoing scientific exploration of the drug’s impact on users’ cognition.

Mr. Nicholas Jackson from the University of Southern California, Dr. Joshua Isen from the University of Minnesota, and colleagues tested three hypotheses that should all be confirmed if marijuana causes cognitive impairment that persists beyond the period of acute intoxication. Their results did not support any of the hypotheses. Instead, their evidence suggests that some children’s genes and family environments set them on pathways that lead both to the use of marijuana and to IQ decline.

A Pattern of Deficits

Mr. Jackson, Dr. Isen, and colleagues analyzed data from two long-term studies of twins: the NIMH-funded Risk Factors for Antisocial Behavior (RFAB) study from Southern California (789 children) and the NIDA-funded Minnesota Twin Family Study (MTFS; 2,277 children). In each study, children took a baseline IQ test involving Wechsler intelligence scales at ages 9 to 12 years, before any began using marijuana, and a follow-up test at ages 17 to 20. In the interim, RFAB participants completed biannual questionnaires about marijuana use, and MTFS participants completed self-reports at 3-year intervals.

Altogether, about 42 percent of the children used marijuana at least once in the decade between the two IQ assessments. These teens lost IQ points compared with their nonusing peers on the Vocabulary section of the Wechsler intelligence scales (3.4 to 4.0 points, on average) (see Figure). The marijuana users in the MTFS also fell behind on the Information section (2 points, on average) (RFAB participants did not take this subtest). On the four other subtests examined—Block Design, Matrix Reasoning, Picture Arrangement, and Similarities—The marijuana users kept pace.

The two subtests on which the teens who used marijuana lost ground measure knowledge acquired through experience and learning. The remaining sections, on which the marijuana users scored on a par with their nonusing peers, measure the ability to reason abstractly and solve problems. These results indicate that the marijuana users accumulated less information than the nonusers during the decade between assessments, but run counter to the conclusion that the drug damaged brain cells that underlie cognitive processing.

Figure. Teens Using Marijuana Show Declines in Verbal IQ Marijuana use between the ages of 9 and 12 and 17 and 20 was associated with declines of 3.4 points in verbal IQ in the MTFS and 4 points in the RFAB. Researchers propose that adverse familial factors give rise to attitudes and behaviors that both promote marijuana use and hamper the accumulation of knowledge.
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The Three Hypotheses

The researchers posited that if marijuana use causes users’ IQ to decline, then:

  • Marijuana use should precede any declines.
  • Heavier users of the drug should incur greater declines.
  • Where one twin in a pair uses marijuana and the other doesn’t, the one who uses should subsequently develop a lower IQ relative to the other.

When the researchers examined the first hypothesis, they found that children who would start using marijuana during their teens had IQ scores on par with future abstainers when assessed at the start of the RFAB study (age 9 to 10). Contrary to the hypothesis, however, future users of marijuana in the MTFS already showed an IQ disadvantage at age 11 to 12.

The data from the twin studies didn’t bear out the second hypothesis, either. The researchers compared IQs of study participants who reported having used marijuana at least 30 times or daily for at least 6 months, to those of participants who had also used the drug, but fewer than 30 times. Both groups had IQ declines of the same magnitude.

Nor did the evidence support the third hypothesis. Twins who were discordant for marijuana use (i.e., one used and the other didn’t) had parallel IQ trajectories. This finding suggests that the twins’ IQ was affected by factors that twins share in their genes or family background, rather than factors in which they differed (e.g., drug use). A further analysis, comparing the impact of marijuana use on fraternal versus identical twins, suggested that family-wide environmental influences are more decisive than genes for determining IQ trajectory.

A Marker for Delinquency

Dr. Isen’s and Mr. Jackson’s team found that marijuana users experienced IQ declines in the teen years, but that the drug appeared not to cause them. What, then, did cause them?

The researchers propose that marijuana users fall behind in IQ because they have experienced adverse familial influences that decrease their motivation to learn and predispose them to engage in delinquency and deviant behaviors, including substance use. These negative attitudes and behaviors impede the normal accumulation of vocabulary and information.

Dr. Isen says, “We suspect that children's delinquent proclivities, and the family-level characteristics that underlie them, drive IQ decline.” For most of these young people, marijuana use is one deviant behavior among others, and their generally under-socialized behavior and delinquency, rather than marijuana use, affects their IQ trajectory.

In support of this view, the researchers found that, in the twin studies, marijuana use appeared to be associated with IQ decline not by itself, but because it is linked to other substance use. Thus, when marijuana use was the only substance use included in the analysis, it correlated with IQ decline. However, when the researchers adjusted the analysis to distinguish between the effects of marijuana use and those of binge drinking, the relationship between marijuana use and IQ:

  • Disappeared in the MTFS.
  • Persisted but was attenuated in the RFAB.

Taken singly, those findings indicate that a teen’s overall substance use, rather than marijuana use, is a more significant contributor to IQ trajectory. Taken together, the findings point to delinquency as the underlying source of these relationships. The key observation is that separating out the effects of binge drinking abolished the association between marijuana use and IQ when binge drinking was a deviant behavior, but not when it wasn’t. MTFS participants were 17 to 18 years old at the time of their last assessment, an age when both marijuana use and binge drinking are markedly deviant; RFAB participants were 19 to 20 years old, an age when marijuana use is still considered deviant, but binge drinking is more or less normative.

Dr. Isen comments, “Binge drinking did not reduce the IQ−marijuana association in the RFAB study because alcohol use is no longer deviant in college-age populations. If we had measured binge drinking in these participants at ages 17 or 18, then it would have likely reduced the association between marijuana use and IQ decline.”

Dr. Isen says that the time of onset of IQ decline in the MTFS, between 11 and 12 years of age, is consistent with the hypothesis that delinquent tendencies stemming from adverse family backgrounds are responsible. He explains, “Delinquent behaviors cannot really unfold until middle school, because there is far less opportunity to cut class and affiliate with deviant peers in elementary school.”

Stay Tuned

These findings contrast but are not entirely inconsistent with those of an earlier study that linked teen-onset regular marijuana use to IQ deficits in the fourth decade of life (see Early-Onset, Regular Cannabis Use Is Linked to IQ Decline). The researchers say that although their evidence indicates that marijuana exposure does not cause persistent loss of intellectual function up to age 20, prolonged regular exposure for decades might do so.

Dr. Harold Gordon, a program official within NIDA’s Epidemiology Research Branch, says, “This is the first longitudinal study to examine IQ and marijuana use in teenage twins. By looking at twins, the researchers could examine siblings who had the same genes and home environment. This allowed them to better control for these factors to determine whether they were associated with the IQ changes seen in marijuana-using teens.”

Dr. Gordon says, “We need additional research on this topic to further understand the relationship between marijuana use and cognitive function, especially memory. We expect that the NIH-funded Adolescent Brain and Cognitive Development (ABCD) study, which is the largest long-term study of brain and cognitive development in children across the United States, will address many unanswered questions about the impact of marijuana and other drugs on the developing brain.” The ABCD includes the study of twins, and the ABCD twin hub is based at the University of Minnesota.

Even if further research discloses no direct link between marijuana and IQ declines, the researchers caution teens against using marijuana. “While marijuana use in adolescence may not be causal of lower measured intelligence, this does not mean that heavy use in adolescence is problem free,” says Dr. Isen. “There are real health risks associated with all drugs, including marijuana.”

This study was supported by NIH grants directed by Dr. William Iacono (DA013240, DA005147, and DA036216) and Dr. Laura Baker (MH58354).


Jackson, N.J.; Isen, J.D.; Khoddam, R.; Irons, D.; et al. Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies. Proceedings of the National Academy of Sciences 113(5): E500-E508, 2016. Abstract

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If the evidence applied to this study is accurate how would it account for people such as myself. Wile to old now to qualify for said study during the same ages and time frames in my life I used almost daily. I was advanced two grades ahead of my peers before accelerated classes was even normalized in school. I graduated school with a 4.333 GPA. Down side I was never taught the basic social skills one learns from peers of the same age group. Even more to the point growing up in the "Just say no era " Nancy Reagan. The idea was if you know you can do something you did not have to prove it to anyone but yourself. Some of us took that quite literally accross the board. That's not the case and some may be dumbing down intentionally to avoid others expectations and the responsibility that comes with.

Response from Dr. Joshua D. Isen: Teenagers engage in marijuana use for different reasons and in different contexts. The majority of daily marijuana users follow a delinquent, rebellious lifestyle more generally. This means they are likely to violate other rules and social expectations, such as skipping class (truancy), neglecting to do homework, and failing to adequately prepare for college. It is possible that your situation was very different from the typical circumstances faced by heavily using adolescents. Perhaps you did not engage in many rule-breaking behaviors besides marijuana use, and thus it was easier for you to stay out of trouble and perform well in school.

Given the rather broad temporal parameters of this particular study's pretest/post-test design, I would be curious as to which particular data set you are analyzing. Is it longitudinal information from the DOJ funded Johnston, O'Malley and Bachman "Monitoring the Future" study (aka "The National Youth Survey")?
Interesting and entirely plausible findings, nonetheless.
Thank you for your time.
R.A. Castro
Southeastern Louisiana University
Department of Sociology and Criminal Justice

Response from Dr. Joshua D. Isen: We used two longitudinal samples of twins from Southern California and Minnesota. Participants were first tested at ages 9-12. If I'm not mistaken, participants in the National Youth Survey were recruited no earlier than 8th grade.

How many of the subjects were long-term heavy users? It is hard to imagine that young people who have tried the drug once, 30 times, or even for six months would have measureable effects on the brain. In your judgment is the study sufficiently sensitive to assess the impact of long-term heavy use?

Response from Dr. Joshua D. Isen: We agree that using marijuana 30 times does not adequately capture long-term heavy use. This is a limitation stemming from the fact that we only asked dichotomous questions to RFAB participants, and the cutoff for one item was "at least 30 times". We believe that the Minnesota Twin Family Study (MTFS) is better positioned to identify long-term heavy users - 186 MTFS participants used marijuana on a daily basis for the past 12 months or during a prior 12-month period. (99 RFAB participants smoked marijuana every day for at least 6 months.) We also asked MTFS participants to provide a continuous number of lifetime uses (ranging from 0 to 999). Among those who endorsed smoking marijuana on a daily basis, the median number of lifetime uses was 400. This seems like an adequate dosage to detect any putative effects of marijuana on the brain.

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