Study on twins shows no link between cannabis use and QI decline, family and sociocultural environment is to blame


Medical literature is full with studies suggesting a link between marijuana use and lower cognitive abilities. However, it seems that none on these studies has put a finger on the real cause of this decline, so many doctors are still betting on a neurotoxic effect of cannabis.

A recent study on twins by Nicholas Jackson, psychologist at the University of Southern California, suggests the contrary – lower intellectual abilities, discovered in marijuana users, are explained rather by their family and sociocultural environment (1).

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A New Zealand study published in 2012 that became reference among doctors (2). More than a thousand individuals were followed from birth until their 38 years. Comparing old early regular consumers (who started using before age 18) and late consumers (having started after 18 years), the authors showed diminished cognitive abilities in adulthood among former teenage consumers. They concluded that cannabis would have a toxic effect on the brain development of adolescents. However, despite the statistical strength of their results, they did not prove the neurotoxic role of cannabis in this decline.

To clarify this issue, Nicholas Jackson and his team have studied two independent cohorts of over 3,000 twins, whose intellectual capacities were evaluated in childhood and adolescence through intelligence quotient (IQ) tests. Cannabis use was in turn evaluated through a questionnaire.

Without surprise, marijuana consumers saw their IQ score drop by 4 points between childhood and adolescence compared to non-consumers. The researchers then compared the evolution of this score in which one of the twin siblings smoked, and the other not. If the substance had a neurotoxic effect, only the twin smoker should have a lower IQ.

Or this is not the case! The IQ of these twins suffers the same decline, regardless of their respective consumption. How to explain this?

Since the twins share the same cultural and family environment, the authors suggest that the real cause of the intelligence decline is the family environment, which has the same effect on both twins at once.

“The interest of this study is to highlight the importance of sociocultural factors on IQ scores, independently of cannabis use,” says Alain Delvaux, addiction psychiatrist at the Sainte-Anne Hospital in Paris.

Is cannabis definitively exonerated in cognitive decline? No. Because in this study only the intelligence quotient was evaluated. Or, cognitive abilities include both the intelligence and the executive functions (planning, organization, adaptation to the environment, etc.).

“Nothing says that the cognitive abilities are not affected by marijuana use, insists Alain Delvaux.  The good news, however, is that over a decade period, cannabis doesn’t seem to have any effect on the intelligence. It remains now to confirm this result during a longer term, twenty-five or thirty years”(3).

Marijuana is legalized in several places and the new Prime minister of Canada is also favorable for that. Sources show documented use of cannabis for medical purposes in China still in 2627 BC, but the subject is still very controversial.

(1) NJ Jackson et al., PNAS. doi: l0.1073 / pnas.l516648ll3, 2016.
(2) M. H. Meier el al., PNAS. 109, e2657, 2012.
(3) G. Cariou, La Recherche, N’509, March 1, 2016

Disclaimer: This article reflects only the opinion of its authors. Its full text is available here. It is about one study and not conclusive. There are probably multiple factors playing a role in what is the actual  impact of cannabis use (including age when started, socio-economics, other health conditions – physical or mental, other medications, of course how much is consumed etc. even nutrition could possibly alter the outcome).

It does seem logical and consistent with current science that when something really affects the brain/CNS, it probably is harmful at some dose level. In CNS research it is truly difficult to avoid secondary effects, so caution is crucial (especially keeping in mind that  marijuana quality has changed over the years and not all historical knowledge is totally valid for today’s products).  However, it does not mean that it is impossible to find a range or situation, where the benefits outweigh the risks. To say that socio-economic background is the actual reason for the IQ difference, looks equally like a premature conclusion. Whether there would be a link between cognitive function and marijuana specifically, is just not yet clear.  We should be careful not to advocate either way, before there is a whole body of serious research pointing to key factors and outcomes. The article is one view about the subject and the debate – and hopefully  research in this direction – will continue.