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Twin Study Debunks Myth That Cannabis Lowers IQ

May 10, 2021 in General News

Does Cannabis use Lower IQ?

One of the most misleading myths about regular cannabis consumption is that it lowers the cognitive functioning of users. This falsehood has been a mainstay of anti-cannabis propaganda for many decades. Its so-called “scientific data” is based on a 2012 study that tied cannabis use by adolescents to a reduction in their IQ and executive functioning scores. In this article I will cite the original study and explain the results of the latest identical twin study which debunks it.

2012 Study: Persistent Cannabis Users Show Psychological Decline from Childhood To Midlife

The study appeared in the August, 2012 online edition of Proceedings of the National Academy of Science of the United States of America. The lead author Madeline Meier, PhD, of Duke University, Durham, NC, collaborated with researchers in New Zealand.

The purpose of the study was to see if there was a correlation between persistent cannabis use and neuropsychological decline in adolescent-onset cannabis users.


Participants consisted of 1,037 members of the Dunedin Study, in New Zealand, who were tracked from birth in 1972-3 to age 38. Cannabis usage data was collected via interviews with the participants at ages 18, 21, 26, 32 and 38. Neuropsychology testing was administered at age 13 before the initiation of cannabis usage through age 38 after persistent cannabis use had developed.


The results of the study found that participants who began smoking cannabis before age 18 and continued into adulthood lost an average of 8 points on IQ tests taken between ages 13 and 38. They demonstrated deficits in memory, learning and executive functioning. Those who stopped smoking cannabis as adults did not recoup their full cognitive function.

There was no similar decline in cognitive functioning of participants who became regular cannabis users as adults. This suggests that cannabis has a deleterious impact on the brains of adolescents. The study authors believed that the policy efforts of governments must be to prevent the use of cannabis in adolescents.

Unfortunately, other similar studies came to the same conclusion and it became accepted by the scientific community that cannabis use causes cognitive decline, especially in teens. There is only one problem with that conclusion; correlation is not the same as causation.

The more accurate deduction should be that some of the genetic and environmental factors which negatively impact cognitive function may increase an individual’s propensity for using cannabis.

Why did these two studies reach such different conclusions? It primarily has to do with the different approaches used by the researchers.

The 2012 study demonstrates a correlation between cannabis use in teens and lower IQ scores in young adulthood. However, it cannot prove whether cannabis usage is responsible for the changes in IQ. Therefore, it is a distinct possibility that other factors caused both the changes in IQ and the increased likelihood of cannabis usage. These include lower socioeconomic status, tobacco use and chronic pain which can be linked to both cannabis use and lower IQ scores. Some researchers even attributed an increase in cannabis usage in teens with their association with peers who are a “bad influence;” those who skip classes, don’t do their school work and abuse other more dangerous substances.

That brings us to the 2019 identical twin study which is well-equipped to look at causal relationships.

Identical Twin Study Debunks The Lower IQ Myth

A 2019 longitudinal study using identical twins was published in Drug and Alcohol Dependence on January 1, 2020. The study, Investigating The Causal Effect of Cannabis Use on Cognitive Function with a Quasi-Experimental Co-Twin Design, was conducted at the University of Colorado at Boulder in their Department of Psychology and Neuroscience and the Department of Psychiatry.


The research team tracked cannabis use and cognitive function from adolescence through early adulthood using 428 sets of twins. Cannabis users were compared to their non-using twins to determine if cannabis use could be the attributed to causing neurotoxic effects on the brain. Most of the cannabis consuming participants in the study reported occasional but not daily use.

Lead author, Dr. Jessica Megan Ross, explains why using identical twins who grew up together in the same household is such a good study model. Identical twins share 100% of their DNA and the same environment, which allows researchers to control for both genetics and  the many environmental factors.


Dr. Ross had this to say about the results:
“Our study did find significant associations between cannabis use and cognition phenotypically. However, when we controlled for genetics, shared environmental factors between twins, and other substance use, these significant associations disappeared. Importantly, abstaining from cannabis use offered no protection from cognitive decline for the non-cannabis using twins.

Previous Twin Studies Corroborated the 2019 Study Results

Several twin studies conducted in 2004, 2016 and 2017 corroborated the latest twin study results. A 2018 literature review published in JAMA Psychiatry analyzed 69 cross-sectional studies with almost 9,000 participants. The outcome demonstrated a small yet significant reduction of cognitive function in adolescents and young adults who reported frequent recurrent cannabis use. However, and most importantly, when this group abstained from cannabis use for more than 72 hours, the effect of cannabis use on cognition decreased to non-significant levels.

Furthermore, the 2019 study included additional testing measures for assessing executive function such as response inhibition, working memory and set-shifting. It also conducted intelligence testing for participants before the start of cannabis usage from adolescence through young adulthood. None of this was included in previous studies.

 Limitations of the Twin Study

Not many of the participants in this study were heavy cannabis users. Dr. Ross viewed this as a limitation of her study. She agreed that it would be important to see if she could replicate the findings using twins who were heavy cannabis users.

More research is definitely needed to learn about how cannabis usage affects the human brain in the long term. The good news is that current research points to evidence that cannabis usage will not harm your intelligence.

Sources:, Cannabis IQ Drop is A Myth, New Twin Study Confirms, Emily Earlenbaugh, Dec. 27, 2019, Investigating The Causal Effect of Cannabis Use on Cognitive Function with a Quasi-Experimental Co-Twin Design, Jan. 1, 2020, Large Longitudinal Study Documents Marijuana-Associated Cognitive Decline from Early Use, Oct. 4, 2012, Study Finds “Little Support” for Cannabis Impacting Cognitive Abilities, Dec. 12, 2019

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