Study Reveals New Findings Concerning Cannabis and IQ

One of the most prevalent cliches and stereotypes cannabis users get labelled with is that they’re stupid. That they’re less intelligent than the general population. “It’s called dope for a reason”, detractors often say, “smoking dope will make you dopey”.

Back in July 2012, a study was published in the scientific journal Proceedings of the National Academy of Sciences of the United States of America which seemed to back up those claims.

Researchers followed individuals from the age of 13 to 38, performing a series of interviews in which they asked participants about their use of cannabis, and used IQ tests to determine the level of ‘intelligence’ of those people who had been regular cannabis users, both before and after the onset of use. They found that in 5% of those participants, their use of cannabis seemed to have had a negative effect on their scores. The results suggested that the heaviest users could lose as many as 8 IQ points.

Newspaper reports at the time were typically hyperbolic, with the Telegraph declaring that “Teenagers who regularly smoke cannabis are putting themselves at risk of permanently damaging their intelligence”. Even as recently as last year, articles were still appearing in widely-read publications, using this research as proof of the claim that cannabis makes you stupid.

In February 2015, Forbes ran an article which reiterated the claims of the Telegraph, and warned that adolescent use of cannabis, by lowering IQ, would not only make you stupid, but poor as well – “…individuals with an IQ of 110 have an average net worth of $71,000 and individuals with an IQ of 120 have an average net worth of $128,000. It looks like smoking pot can lower your tax bracket.”, they claimed.

What’s interesting is that even before the Forbes article, other researchers had already begun to question the validity of the conclusions drawn by the original study. In August 2012 (just one month after the study was published) Time Magazine interviewed Dr Carl Hart, associate professor of psychology at Columbia University, as part of an article on the subject. Already, he was doubtful. He pointed out that “when you compare these people’s scores to a normative database on a wide range of domains including executive function, memory and inhibitory control, they score dead smack in the middle, in the 50th percentile.”

There were also questions as to why the original study did not include data on whether the participants were employed, and whether they were able to function normally in their families and in society – factors which are important to give context to the study and to show whether the apparent drop in IQ had real-world implications.

“There are also other factors”, Time pointed out “such as child abuse or other trauma — that might lead people to seek escape in heavy marijuana use and could also affect brain function.” These factors were not examined by the authors of the study.

So, as usual, opinions varied. Now though, there has been a fresh look at the issue. Published in the Journal of Psychopharmacology, this new study took essentially the same approach as the one published in 2012, but with a few key differences. Firstly, the sample size was larger – 2,235 compared to 1,037. The participants had also taken an IQ test at age 8, 5 years before those in the original study and, importantly, before any of them had tried cannabis.

The initial findings seemed to reinforce those published in 2012 – those teenagers who had used cannabis even infrequently scored lower on subsequent IQ tests than was predicted by their score at age 8. They even performed worse in their GCSEs. Crucially though, the research team didn’t stop there. They went back to the data and examined factors other than cannabis use. Once they had statistically adjusted their findings to account for differences such as alcohol use, tobacco use, childhood behavioural problems, and mental health symptoms, cannabis use no longer predicted lower IQ scores. It would appear, then, that cannabis use doesn’t make you stupid after all.

It’s worth pointing out though, that whilst the new study improves on the first in key areas, it also has its own limitations. The subjects of the first study were followed up well into their 30s, where this one only had data on its subjects up to the age of 16. So it could be that the decline in IQ starts later in life, although this seems unlikely.

It’s also clear that a drop in IQ did take place, even if that drop can’t be attributed to cannabis use alone. This study does not attempt to provide an answer as to why that is the case, and should not be taken as proof of anything. As we have seen, one of science’s greatest strengths is its ability to review and re-examine, and we could be back here in another 4 years looking at another study with more concrete conclusions, be they similar or completely different to those drawn by this one.

There is one thing that definitely can be taken from this new research, though, and it was elucidated perfectly by Claire Mokrysz, a PhD student at University College London who was involved in this study. Writing for the Guardian last week in the wake of the results being published, she said:

“Our study is by no means definitive, but it does highlight that we should all be more cautious when jumping to conclusions about the harms of a drug before we have strong evidence either way. Overly forceful conclusions about the potential negative effects of cannabis are unscientific and based on an incomplete evidence base. This can lead to the unfair marginalisation of teenagers who use cannabis, which is the last thing we would want, given that this group is likely to include some of the most vulnerable in society.”

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