Study finds adolescent marijuana use reduces IQ: Parents and Politicians rejoice
In many ways, smoking your first joint is an integral part of growing up. Whether peer pressure sucked you in, or you just wanted to try something new, marijuana use is skyrocketing among the youth, with 4% of students in grade 10 and 6.6% of students in grade 12 using marijuana daily in 2011. Marijuana, venerated in movies and by rappers, is the world’s highest consumed illicit drug, due to the preconception that the drug itself is harmless. Indeed, anecdotal tales abound amongst us all about the friend of a friend who was stoned throughout his classes and still got A grades. Recently a report in the Proceedings of National Academy of Sciences (PNAS) on persistent marijuana and IQ decline cause a large number of science journalists to jump on the article, politicians to look down their glasses, wag a finger and say “I told you so” and potheads the world over to go “Duuuuuuuude”.
The study in question arises from New Zealand, and describes data obtained from participants of the Dunedin study, which followed 1037 individuals from their birth (in 1972/73) until they turned 38 (2011/2012). To study the effect of marijuana use on their mental processing abilities, these individuals were given an IQ test at the age of 13- hopefully before they had the chance to smoke up – and then regularly for nearly 20 years at the ages of 18, 21, 26, 32, and 38. At every test, persistence in use was measured, until the final test at the age of 38. What the authors saw was that persistent use by adolescents, ie having been found to have used marijuana in 3 or more of the ages where IQ tests was administered, resulted in the highest IQ drop of up to 6 points compared to those who have never done the drug (who actually increased in IQ by 0.8 points). Similar IQ drops were seen in other users as well, although not to the same levels as persistent users. In those adolescents who had managed to get over their dependence, their IQ levels never recovered to pre-marijuana levels; a similar decrease was not seen in users who only started using marijuana after they became adults (age 18+). The reason this study received so much attention was due to the large number of youth studied, as well as strict ways to ensure no confounding variables- such as other drugs/alcohol dependence, as well as controlling for years of schooling. While it can be argued that IQ is not an ideal measure of one’s intelligence-indeed factors such as wealth, height and proper food are correlated with higher IQs- the authors mention that persistent adolescent users also have decreased attention and memory in daily life settings.
So what does this all mean? Is the use of marijuana bad? Was that first joint we smoked in high school to impress a girl affecting our chances at getting into grad school now? As with all science, data involving such human subjects pose a real problem in coming up with a definite conclusion. A number of questions can be asked- was the wealth of the participants controlled in the study? Wealthy adolescents can buy more weed, but wealth also correlates with a higher IQ. Where does that leave us? Were other extraneous conditions- such as family life and/or depression controlled? The data from the adult groups only has about 50 participants in all compared to the initial 1037 participants, suggesting marijuana persistence might be more rare than expected.
One thing is certain. The brain of the adolescent already has a lot of things to take care of- puberty or navigating high school for instance. While biologically, it is possible that marijuana or other drug use could cause changes, much more research in this field is needed before jumping to any conclusions. Not that such trivial details would stop our politicians.
1) Johnston LD, O’Malley PM, Bachman JG, & Schulenberg JE. (2012). Monitoring the Future national survey results on drug use, 1975-2011. Volume I: Secondary school students. Ann Arbor: Institute for Social Research, The University of Michigan, 760 pp.
2) Meier MH, Caspi A, Ambler A, Harrington H, Houts R, Keefe RS, McDonald K, Ward A, Poulton R, & Moffitt TE (2012). Persistent cannabis users show neuropsychological decline from childhood to midlife. PNAS 2012 Aug 27.