HomeMedicalTwins study shows cannabis does not lower IQ

Twins study shows cannabis does not lower IQ

IQ levels in adolescents are completely unaffected by cannabis use, confirms an extensive medical study.

The research by the Proceedings of the National Academy of Sciences – led by Madeline H Meier – delivered a u-turn on the study’s original findings in 2012 which claimed prolonged use of recreational cannabis could permanently lower intelligence quotient (IQ).

However, the same team revisited the study with fresh data and material before concluding there was no evidence to suggest IQ and cognitive ability could be diminished through long-term use of marijuana.

The report determined that IQ was more likely to be dictated by genetics than cannabis.

Findings from the original study were reversed following an improvement to how the research was processed.

Upgraded report

The 2012 report was taken from research in Meier’s study with Otago University in Dunedin which gathered data from more than a thousand New Zealanders born in 1972 and 1973. The upgraded report used data from twins from England and Wales.

The twins were part of a longitudinal study which tracked and tested them at the ages of five, 12 and 18. Researchers harvested data on IQ levels at the 12 and 18 year studies.

The test results made a dramatic difference to the conclusion of the study – hence the u-turn on the 2012 report.

Concluding her findings, Dr Meier said: “Compared with adolescents who did not use cannabis, adolescents who used cannabis had lower IQ in childhood, prior to cannabis initiation, and had lower IQ at age 18, but there was little evidence that cannabis use was associated with IQ decline from age 12–18.

“For example, adolescents with cannabis dependence had age-12 and age-18 IQ scores that were 5.61 and 7.34 IQ points lower than adolescents without cannabis dependence, but adolescents with cannabis dependence did not show greater IQ decline from age 12–18.

Executive functions

“Moreover, adolescents who used cannabis had poorer executive functions at age 18 than adolescents who did not use cannabis, but these associations were generally not apparent within twin pairs.”

As an example, the report cited same-sex twins who used cannabis more frequently than their co-twin performed similarly to their co-twin on five of six executive function tests. The one exception was that twins who used cannabis more frequently than their co-twin performed worse on one working memory test.

“Short-term cannabis use in adolescence does not appear to cause IQ decline or impair executive functions, even when cannabis use reaches the level of dependence,” concluded the study.

“Family background factors explain why adolescent cannabis users perform worse on IQ and executive function tests.”

The report was revived this week after Mike DeWine, governor of Ohio, announced he was firmly against state-wide legalisation of recreational cannabis, claiming reports suggested it caused cognitive degeneration.

“We have evidence now that if someone is using marijuana and their brain is still developing, which can be up until their twenties, if they use it regularly, they can have a significant, permanent drop in IQ,” he slammed in this Leaf Desk article.

“Some people look at marijuana as just some benign drug, and it’s really not.”


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