Teenage cannabis use is believed to lead to a decline in intelligence quotient. But a study of twins suggests the link is not so clear.
If you grew up in the ’80s or’ 90s, you must have come across some TV clips with the following message: Cannabis makes you dumb. In the United States, Republican candidates, like Donald Trump, are even making the fight against cannabis consumption a major focus of their campaign.
As an argument, these detractors often emerge from a study from Duke University (Durham, North Carolina) conducted in 2012, which found that heavy marijuana use during adolescence and adulthood is associated with a decline in Intelligence Quotient (IQ).
Results strongly criticized by the scientific community and again called into question in a study unveiled on Tuesday.
No drop in academic results
In this new work published in Proceedings of the National Academy of Sciences (PNAS), American researchers analyzed data collected during two studies conducted on large cohorts of twins. They included a total of 3,066 participants who were given a battery of intelligence (IQ) tests between 9-12 years old and again between 17-20 years old. Each time, these scientists compared a twin who did not use marijuana to his brother who used it.
Result, regardless of the cohort, the study leads to the same conclusion, explain the researchers. They found no evidence that adolescent marijuana use leads to a decline in intelligence and therefore IQ.
After adjusting for a range of confounding factors (maternal health, mental health, other substance use), the researchers found that “cannabis use at age 15 does not lead to lower IQ scores in children. adolescent girls or even poorer school performance ”.
Specifically, the marijuana users lost around four IQ points during the study, but their abstinent twin siblings showed a similar trend of decline!
“These results suggest that here, the decline in mental acuity was due to something other than cannabis,” said Nicholas Jackson, lead author of the study and Professor at the University of Southern California (Los Angeles) . Our data lead us to believe that this drop in IQ is linked to something else, present in the environment of these young people, and which leads to a loss of gray matter, ”he adds.
“The consumption of cannabis at low levels is therefore not in itself a causal link with cognitive impairment, nor with degraded life trajectories,” the team concludes.
However, it should still be noted that heavy cannabis use involves risks. A recent study by Northwestern University (Chicago) showed, for example, that frequent cannabis use during adolescence can affect memory in adulthood. And recently, the scientific journal Addiction took stock of 20 years of cannabis literature. The knowledge on the risks available to us is thus increasingly precise (risk doubled from road accident, etc.)
Teens: Smoking #cannabis would not make any less intelligent -Study @PNASNews https://t.co/cuaQgHhDTH pic.twitter.com/GVig9IXXzX
– Pourquoidocteur (@Pourquoidocteur) January 20, 2016
Teenage #cannabis use is believed to lead to IQ decline. But a study published in the journal PNAS on twins suggests the link is not so clear:
Posted by Why actor on Wednesday, January 20, 2016
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