By Max Sargent

Cannabis has long been linked to low IQ, with a general reputation that it somehow makes people less smart. However, the issue is incredibly complex, and we do not yet know what the effects of cannabis on intelligence are. For that matter, we do not yet really know what intelligence is. However, studies are beginning to reveal that although there is a link between cannabis use and IQ, it may not be a causal one.

What Is IQ?

IQ stands for “intelligence quotient”, and serves as a quantifiable way of testing and measuring a person’s intelligence. It is considered by many a controversial measure, but more on this shortly.

IQ is measured by the famous IQ test. There are many versions of the IQ test, and some go by other names. However, they tend to be fairly standardised these days. They may test abstract reasoning, logical reasoning, verbal reasoning, general knowledge, and more.

Around two-thirds of participants will score between IQ 85 and IQ 115. About 2.5% will score above 130 or below 70.

What Does IQ Actually Tell Us?

Far from being a pure metric of intelligence, IQ has been examined for being influenced by many different factors, few of which directly tell us about the fundamental functions of someone’s brain, but rather about a host of interacting physical and environmental factors. What’s more, “intelligence” itself is an elusive concept, which makes measuring it very difficult.

While IQ tests may be a valid measure of a limited type of “academic intelligence”, they fail to tell us much about social intelligence or creativity. Therefore, even by placing stock in IQ, we can observe our own biases regarding intelligence and the culturally ingrained values we have surrounding it.

Moreover, the types of cognitive ability IQ measures have been strongly linked to environmental factors. While this does not mean that they don’t measure intelligence—there is no reason intelligence shouldn’t be a sociobiological interaction—it does mean that it brings biases with it, and perpetuates them. For instance, IQ tends to reward people like those who created it—middle-class white men.

Studies show that differences in average scores between different racial groups can be attributed to socioeconomic factors[1], rather than genetic ones. Indeed, a rapid closing of the gap[2] is clear evidence that environmental, rather than inherent, differences are the cause of such findings. What does this tell us? That IQ can be taken as a measure of someone’s background[3] as much as their intelligence.

What Does IQ Actually Tell Us?

  • Why Is This Important?

A little background of the flaws associated with IQ is important, as the correlation between cannabis use and reduced IQ may depend on these flaws. Simply put, cannabis use is related to lower IQ. However, upon closer inspection, it appears that cannabis use and low IQ may share similar causes, rather than one causing the other.

Does Weed Lower IQ?

Cannabis use, especially in adolescence, has long been said to cause a reduction in IQ over time. There are many studies that support this hypothesis. However, when you delve a little deeper, it transpires that cannabis may not be the cause after all.

Evidence That Marijuana Lowers IQ

It does seem to be the case that cannabis use in adolescence and a lower IQ are linked. For instance, a study by Meier et al.[4] tested the IQ of 1,037 participants at age 13, and again at 38. Between these ages, they were interviewed at 18, 21, 26, 32, and 38 in order to ascertain their cannabis use.

It was found that there was a statistically significant decline in IQ in users who were deemed to have “persistent” cannabis habits. Moreover, the decline was concentrated among those who began using cannabis during adolescence. Perhaps concerningly, it was also observed that cessation from cannabis use—even for long periods of time—did not reverse this decline. It was concluded that cannabis has neurotoxic effects on the developing adolescent brain.

More recently, a longitudinal study by Power et al.[5] found that persistent cannabis use in adolescence is linked to an average drop of two IQ points. This study screened 2,875 papers, and eventually conducted a meta-analysis on seven of them. These seven papers contained 808 cases and 5,308 controls. It is likely that this is the largest longitudinal meta-analysis regarding cannabis’ effects on IQ to date.

It found, as other studies have, that cannabis use in adolescence was linked to a drop in IQ, most prevalently to verbal IQ change.

However, while this study draws a clear correlation between cannabis use and IQ decline, it cannot point to cannabis being the cause of this decline. One important facet of all scientific testing: correlation does not equal causation.

There are many studies demonstrating that although cannabis use and declining IQ are connected, this does not mean that cannabis is necessarily the cause.

Evidence That Marijuana Lowers IQ

Evidence That Cannabis Does Not Cause a Low IQ

There is increasing evidence that the observed decline in the IQ of cannabis users has causes that predate cannabis use. Not only this, but cannabis use is thought to share these causes, explaining the strong correlation between what may be two effects of more difficult-to-identify causes.

Studies on twins are incredibly useful ways of distinguishing causes. Though averages can be gleaned from large populations, it can be difficult to apply these to any one individual, as you can’t know what that individual would have been like in any of their contingent futures.

In the context of cannabis, while it is possible to say that chronic smoker X has seen a two-point decline in IQ while non-smoker Y has not seen a decline, as persons X and Y are different, it’s hard to determine if person X would have seen this decline had they never smoked.

Studies on identical twins go some way to remove this problem, as one can work as the other’s control group. Due to genetic similarities and similarities in upbringing (this is the most contested aspect of twin studies—small differences can have big outcomes), it is deemed easier to isolate causes for differences between twins. In the case of cannabis use, if one twin uses cannabis and has a lower IQ than the other who doesn’t, and this is observed across twin pairs, the conclusion that cannabis use causes low IQ could be drawn.

Studies show this is not the case.

A longitudinal study by Jackson et al.,[6] which included many twin pairs, found that while cannabis use could be linked to a drop in IQ, this did not hold up with the twin pairs. In twin pairs in which one used cannabis during adolescence and the other did not, there was no significant difference in IQ between them at any point during the research.

From this they concluded that other factors, such as socioeconomic factors and familial factors, were responsible for the loss in IQ points, and not cannabis use.

Likewise, another study, again by Meier et al.[7], demonstrates the same findings. There were three important discoveries in this study. First, those who smoked cannabis were observed to have a lower IQ by age 12, prior to cannabis initiation. Second, while their IQs did drop over the proceeding years (during which they used cannabis), the drop was no greater than that observed in the participants who did not use cannabis. Third, the differences observed between users and abstainers were not observable in twin pairs where one used cannabis and the other did not. They too concluded that other factors were behind both cannabis dependency and a lower IQ.

It’s important to note that neither of these studies concluded that low IQ causes one to smoke cannabis.

Evidence That Cannabis Does Not Cause a Low IQ

Though more needs to be done, research seems to show that socioeconomic and environmental factors cause both a decline in IQ and persistent cannabis use in adolescence. Linking back to the beginning, as it has been shown that socioeconomic factors were the cause of racial differences between IQ scores, so too could they explain the differences between those who use cannabis and those who do not.

All that being said, we must not assume that cannabis use does not negatively impact IQ, especially in those who use it when their brains are still developing. It would be foolish to suggest there is no link between adolescent use and decline in IQ, even if the link is not a causal one. Therefore, we suggest not using cannabis until adulthood.

External Resources:
  1. Group differences in IQ are best understood as environmental in origin - PubMed
  2. SAGE Journals: Your gateway to world-class journal research
  3. IQ testing 101 : Kaufman, Alan S., 1944- : Free Download, Borrow, and Streaming : Internet Archive
  4. Persistent cannabis users show neuropsychological decline from childhood to midlife | PNAS
  5. Intelligence quotient decline following frequent or dependent cannabis use in youth: a systematic review and meta-analysis of longitudinal studies | Psychological Medicine | Cambridge Core
  6. Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies - PubMed
  7. Associations between adolescent cannabis use and neuropsychological decline: a longitudinal co-twin control study - PubMed
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