A study on the follow-up of cannabis users, conducted by the University of Duke and published at the end of August, was widely covered by the media. Researchers were clear in their conclusion: “Pot use before 18 harms adult IQ” (in the words of a headline appearing in USA Today). However, after reading the original article, it is possible to notice several points that put this conclusion into perspective, showing how a scientific article, with its strengths but also its weaknesses, can become an “uncontrolled media object”.
This article is a translation of “Cannabis et QI : les média enfumés ?” available at: http://blog.mysciencework.com/2012/10/01/cannabis-et-qi-les-media-enfumes.html It was translated from the French by Mayte Perea López.
On August 27, an article published in Proceedings of the National Academy of Sciences on the neurotoxic effects of cannabis created a buzz. It must be said that it was a timely publication, as a few days before the start of the school year is traditionally when all the media turn towards the young, in order to take stock (again) of their lifestyle (sleep, eating, outings, alcohol, drugs, etc.).
Within 24 hours, radio and TV channels and the press spread the results of this study that showed that smoking cannabis as a teenager lowered IQ in adulthood, irrevocably! Finally, some solid arguments for parents to prove to their kids that cannabis is not a soft drug. But the threat goes much further than mere physiological damage; cannabis use would purely and simply ruin the future of these reckless youngsters. A quote from Madeline Meier, author of the study, was used in a dispatch from the global news agency AFP and taken up in several press articles: “Somebody who loses eight IQ points as an adolescent may be disadvantaged compared to their same-age peers in numerous important aspects of life.” This also gave food for thought to former cannabis users who suddenly asked themselves to what extent the joints smoked in their teen years had already damaged their intellectual capacities!
The strength of this study is its experimental design. Indeed, scientists followed slightly more than 1,000 subjects from birth to about 38 years old. This is called a “longitudinal follow-up”. This extensive, long-term work is rare enough to be highlighted. What’s more, these “1,000 subjects” studied were one of the arguments that contributed to the media success of the article. The French newspaper Le Monde, for instance, mentions an “extensive study” in its supplement Science & Techno issued on September 6. It is true that most of the time, the limits of scientific studies, especially those conducted on humans, are found in the size of the studied group. To obtain a sufficient statistical power, and thus to show effects that are significant and cannot simply be explained by chance, data must be collected on a large number of subjects. This number is always indicated by the authors in their article by the formula “n=”.
What about this article? The description of the methodology clearly shows that the cohort was made of 1,037 subjects born in Dunedin, New Zealand, between April 1972 and March 1973, 1,004 of which were still followed at the end of the study, between 2010 and 2012. Let’s now take the results that were the most widely spread, those concerning the lowering of IQ among “persistent smokers”. To evaluate the dependency on cannabis, researchers recorded the subjects’ consumption at the ages of 18, 21, 26, 32 and 38. Persistent smokers were those who stated in at least three of the five interviews that they smoked at least four days a week. Taking into account the exclusion criteria to measure the evolution of IQ reduced the number of subjects to only 874. It should be added that the size of the different groups is very heterogeneous: 242 people have never smoked, as against 38 persistent smokers, which means less than 5% of the total studied group! Strangely, this fact was pointed out by Pr. Jean-Luc Martinot when he was interviewed for the article in Le Monde, but apparently this wasn’t considered sufficient, as the newspaper headlined the piece “Le QI en fumée” (“IQ goes up in smoke”) and talks about “the significant and permanent lowering of IQ”. Incidentally, this difference in IQ observed among the subjects aged between 13 and 38 would correspond, according to the authors, to a loss of “about 6 IQ points”, as opposed to the 8 point loss that was read/heard everywhere, on the basis of AFP’s original dispatch.
It would also be interesting to question the relevance of using IQ tests. In Europe, these tests are much less used than in the United States, where they are given more value. For that matter, the author of the article had highlighted the importance of these results stating that “we know IQ is a strongly determining factor in access to University, life-long wage earned, access to employment and performance at work”. In a short interview for the Swiss daily newspaper Le Temps (29/08/2012), a neuroscience professor at the University of Geneva reminded us that the loss of a few IQ points could probably not be considered as a major damage to the subjects’ intellectual capacities. He also expressed reservations about the conclusions of the study regarding the permanent nature of these effects. It is true that adolescence is a key period for brain development, but the brain’s life-long plasticity has also been demonstrated, and with regard more specifically to cannabis, other contradictory results exist in the literature.
Of course, the quality of this study must be highlighted; it was published in a very good peer-reviewed journal, so it was evaluated independently before publication. However, it is also important to show how both mainstream and specialized media stepped into the breach. While the public has to face what some people call “infobesity”, it is crucial that the media learn to step back a little when it comes to news wires that they sometimes take on blindly and in a hurry. It is true that scientific time does not necessarily correspond to journalistic time, but we should probably learn to get some distance from a scientific buzz in order to clarify the information and shed a more informed light on it. In an ideal world, we should also accept the idea that data is not as “sensational” as the editor-in-chief would like it to be to make tomorrow’s front page catchier!
About the author:
Doctor of Life Sciences, specialized in cardio-vascular physiology, Stéphany Gardier started her career as a teacher-researcher in Lyon, before joining the University of Lausanne, and then of Geneva. After more than ten years spent between labs and lecture halls, she took off her researcher’s coat to become a journalist and is currently enrolled in the scientific journalism master’s program offered by the University of Paris 7 – Diderot.
Find out more:
"Persistent cannabis users show neuropsychological decline from childhood to midlife", PNAS abstract http://www.pnas.org/content/early/2012/08/22/1206820109.abstract
"Adolescent Pot Use Leaves Lasting Mental Deficits", Duke University press release http://today.duke.edu/2012/08/potiq
"Study: Pot use before 18 harms adult IQ, memory", USA Today report on the study http://content.usatoday.com/communities/ondeadline/post/2012/08/study-pot-use-before-age-18-harms-iq-attention-memory/1#.UGhWeRhhMb2