Last year a study published in the Proceedings of the National Academy of Sciences (PNAS) touted a possible link between adolescent marijuana use and a later decline in cognitive abilities. Analyzing a longitudinal cohort study of 1,037 individuals from Dunedin, New Zealand, researchers from Duke University found evidence that heavy cannabis use beginning in adolescence was associated with a drop in IQ of as much as eight points by adulthood.
The study made headlines last summer, chiefly because it fit right it with common stereotype that smoking pot dulls the mind. Acutely, this is undeniable. Though weed smokers may think they're spouting sage wisdom and intellectual postulations in between munching on handfuls of jellybeans and flamin' hot Cheetos, they really sound more like Bill and Ted before they embarked on their excellent adventure. ("Okay, the lady in that car over there said that Marco Polo was in the year 1275.
It's not just a water sport, I knew it!")
But, until last year's study was thrown into the fray, the collective evidence for marijuana-fueled cognitive impairment over the long term was insubstantial. On average, when not under the influence of the drug, marijuana users appeared just as cognitively inclined compared to non-users. This was demonstrated in both longitudinal and twin studies.
Last year's fresh research re-ignited discussions on pot's safety. Moreover, the researchers even went so far to conclude that cannabis may have a "neurotoxic effect" on the adolescent brain that could permanently lower IQ.
However, an analysis published Monday in PNAS questions the findings of that study. Ole Rogeberg, a senior research fellow at the Ragnar Frisch Centre for Economic Research in Norway, believes that socioeconomic status likely confounded the correlation, causing the Duke researchers to overestimate the link between cannabis and a decline in IQ.
Using a computer model operating under the assumption that socioeconomic factors like education, income, and occupation affect IQ, Rogeberg ran 500 tests with a sample size similar to that from the Dunedin cohort. The model effectively reproduced the estimates from the original study, thus implicating socioeconomic status as a potential confounder.
The Dunedin Longitudinal Study -- which the original study drew its data from -- was created by recruiting 1,037 people born over the course of one year in Dunedin, New Zealand. The city itself is quite a diverse place, so people from all walks of life were recruited into the cohort. With such diversity, it does open the door to uncontrollable variables.
To illustrate his contention, Rogeberg pointed to a 2002 study that was drawn from a less socioeconomically diverse population. It reported no correlation between cannabis use and cognitive decline.
All in all, Rogeberg's analysis raises a valid argument, but it by no means discredits the original study.
However, the original study isn't perfect either. For example, the 8-point IQ decline only materialized in heavy-users of marijuana who started smoking before 18 and reported being extremely dependent on the substance. Only 38 subjects fit this description -- somewhat of a small sample size. Furthermore, it doesn't seem surprising that those dependent on marijuana would show cognitive decline, especially if they had been under the influence around the time that the IQ-test was conducted.
The real takeaway message to be drawn from the original study is one that everyone should already know: consuming excessive amounts of mind-altering drugs is never a good idea, especially at a young age when the brain is in its prime stage of development.
There still isn't a conclusive body of evidence showing that recreational marijuana use causes long-term declines in intelligence, but now that it's legal in Washington and Colorado, the scientific doors are open to larger arenas of research.