Cannabis Reduces IQ

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• The results of the Dunedin study point to a decline in IQ among moderate and heavy cannabis users
• The results can be interpreted in various ways
• The decline in IQ need not necessarily be caused by medical cannabis

Whether or not regular cannabis use lowers IQ is the subject of a heated debate that has been going on for years. Politicians, parents, scientists and, of course, the ever-present media - all of them take part in this debate very passionately (and empathetically).

Recently, markedly contradictory opinions can be seen in various media - for example, in 2013 the American news outlet Fox News published without hesitation an article headlined "Yes, marijuana lowers IQ - studies prove". A year later, by contrast, we have the well-known Washington Post with the headline "No, marijuana use doesn't lower your IQ".

So who is right?

"YES, cannabis lowers your IQ"

The vast majority of reports claiming that cannabis lowers IQ take their data from a single, very influential study - the so-called Dunedin study, led by Madeline Meier. It should be mentioned that this study is to this day one of the highest-quality studies dealing with cannabis use. Many other studies of this type are retrospective, whereas the Dunedin study is a so-called prospective cohort study.

In retrospective studies, the condition of the subjects is known from the outset, and the scientists try to search their past and determine what might have led to that condition. By contrast, prospective studies follow subjects over time and try to identify the presumed consequences of a given behaviour, taking into account possible variable factors that could theoretically influence the presumed consequences.

In the Dunedin study, a full 1037 people were observed, born between 1972 and 1973 in New Zealand, in the city of Dunedin. All the people were observed from birth to the age of 38. This study, just like all other studies, has its shortcomings. Unlike many other research projects, however, the scientists in this study honestly mentioned the shortcomings they were aware of.

The results of the Dunedin study could have been influenced by many factors, but the scientists took into account, for example, the following:

• Acute or residual intoxication with cannabis at the time of questioning

• Tobacco dependence

• Dependence on hard drugs (e.g. heroin, cocaine, amphetamines)

• Alcohol dependence

• Schizophrenia

There are, however, other factors that could potentially have influenced the results. We might ask, for example: is it possible that people who start using cannabis at an early age share certain character traits that could further be associated with a decline in IQ? Today we also know that factors such as industrial pollution, eating habits, stress, traumatic experiences and many others also contribute to a decline in IQ (these factors can also influence the development of dependencies).

Nevertheless, trying to control absolutely all potential variables is simply impractical. In the Dunedin study, the scientists did everything expected of a quality study - they tried to control as many variables as possible while honestly mentioning all the shortcomings.

In the end, the authors of the study examined 6 main hypotheses:

• Cognitive decline: Persistent, long-term cannabis users ultimately showed a more significant decline in cognitive performance from childhood to adolescence compared with people who do not use cannabis.

• Specificity: Are the impairments of the cognitive system limited to a certain area or cognitive domain, or do they extend across several cognitive areas?

• Education: Some other research suggests that the higher the education attained, the higher the resulting intelligence of individuals. Could the reduced cognitive performance of cannabis users be caused by leaving school earlier and pursuing other interests?
• Everyday cognitive function: Does the influence of cannabis on neuropsychological functions carry over into everyday problem-solving? The scientists in this study assume that it does.

• Developmental influence: Cannabis acts more toxically on the still-developing brain than on an already developed brain, so adolescents are particularly susceptible to the negative effects of cannabis use.

• Recovery hypothesis: Here the scientists assume that in former regular users, damage to neuropsychological functions may be reversed if they stop using cannabis.

So what are the results?

• An early start of cannabis use has a negative impact on cognitive functions and IQ in adulthood, and the greater the extent of cannabis abuse, the worse the resulting effects

• In individuals who waited until adulthood to start using cannabis, no negative impact on cognitive functions or on IQ was found

How did abstainers fare compared with users?

This is where the whole issue starts to get complicated.

Abstainers: 242 of the total 1037 study participants reported that they had never used cannabis in their lives. In these subjects, a slight increase in IQ was measured, on average from 99.84 to 100.64.

Occasional users: 479 of the participants reported that they use cannabis occasionally but had never had a problem with dependence. In these subjects, a slight decline in IQ was measured, on average from 102.32 to 101. 25, i.e. by one point.

Problem users: In 38 study participants, a "cannabis use disorder" was identified in at least three of the total of five assessments at the ages of 18, 21, 26, 32 and 38. The decline in IQ in these users was much more drastic - on average IQ fell from 99.68 to 93.93, i.e. by a full 6 points.

Note one particularly interesting fact. Study participants who admitted occasional, non-problematic cannabis use had an IQ on average 2.5 points higher than abstainers. If we look at the subsequent data for both groups at the age of 38, we see that the slight increase in IQ among abstainers and the slight decline among occasional users almost levels out the IQ of both groups - occasional users still have a lead of 0.61 points over abstainers.

In this case, then, a more fitting headline would be "Studies prove: People who do not use cannabis are less intelligent than those who do."

"NO, cannabis does not lower IQ."

Despite abundant criticism, the Dunedin study was very well designed, and the authors honestly tried to take into account many potential influencing factors that could in any way colour the result of the study. Interestingly, most critics of the first study at the same time praise the second, follow-up study by the same authors, which showed that the only negative physical effect of cannabis use is an increased chance of developing gum disease.

Nevertheless, some of the criticisms of the Dunedin study are still worth mentioning:

Socioeconomic factors

This is the explanation proposed by the Norwegian economist Ole Røgeberg. The basis of his view lies in the argument that cannabis use is more common among people of lower socioeconomic status. The demonstrated short-term increase in IQ in people who attain higher education (which is less common among people of lower status) then creates a false impression of a decline in IQ among cannabis users.

Nevertheless, findings from the Dunedin study show that the rate of cannabis use can hardly be predicted on the basis of socioeconomic status (this fact has also been shown by other studies). Cannabis is simply popular across all strata. Moreover, the Dunedin study did not record lower IQ among people from lower socioeconomic strata either.

Personality traits could explain declines in IQ

Michael Daly, a doctor of behavioural sciences, suggests that the decline in IQ could be explained by the connection between personality traits and cannabis use. Daly points to a non-random connection between openness to new experiences and the tendency to try cannabis, based on an analysis of data from a British study observing 6,401 participants.

Interestingly, people who are open to new experiences and have also tried cannabis have also been shown to have better functioning of the cognitive system.

The number of participants is insufficient

Another critic is Dr Carl Hart, who mainly points out the utterly insufficient number of heavy users analysed, of whom there were, after all, a mere 38 in the Dunedin study.

Everyone can interpret the criticism and the praise of this, to date the most significant study dealing with the decline in IQ among cannabis users, in their own way. The truth is that if we look at the study thoroughly, certain shortcomings cannot be overlooked.

For example, the group of users in whom a cannabis use disorder was shown in one of the five diagnoses had by far the lowest IQ. Conversely, in those in whom the disorder was diagnosed twice, the average IQ was identical to that of occasional users and of those in whom the disorder was not diagnosed at all (102.14). How is that possible?

Cannabis, IQ and Twins

As we have already mentioned, it is more or less impossible to take all possible variables into account. Nevertheless, one way to eliminate most of them is a study carried out on twins. Fortunately, we have one such study.

"Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies."compares changes in IQ between twins who either use cannabis or abstain, over a period of 10 years. After taking possible variables into account, the scientists found that cannabis has no effect on IQ in twins. Moreover, in pairs of twins where one used cannabis and the other did not, an identical decline in IQ was demonstrated. This suggests that entirely different factors may be responsible for the decline in IQ - genetics, home environment and so on.

However, even this study, which completely refutes the results of the Dunedin study, has its shortcomings. For example, it pays very little attention to frequency of use, which can have a considerable influence on the resulting decline/increase in IQ. Another shortcoming is that the study compares data from populations in different areas (Minnesota and Los Angeles) and at the same time uses different questionnaires.

Final evaluation

It appears that in adults, cannabis use does not affect IQ in any way. Nevertheless, chronic use has a considerable, albeit reversible, negative impact on cognitive functions. As far as teenagers are concerned, there are still many questions about the extent to which early use negatively affects the development of cognitive functions. What we do know for certain is that the developing brain is particularly vulnerable.

It is therefore possible that there is some truth on both sides of the debating barricade. One is that too early a start of heavy cannabis use can have negative, potentially irreversible consequences. The other is that other factors may also be behind the decline in IQ - if a child uses cannabis very early and often, it is quite possible that other things are happening in their life that can influence both the development of dependence and their IQ level.

However, the results of the Dunedin study and of other well-conducted studies should not be overlooked despite possible variable factors.

The fact that too early and heavy cannabis use among young people could have a negative influence on their development is always important to mention, even though the stigma surrounding medical cannabis is slowly but surely fading.

Author: Jeremy Kossen

Translation: Filip Maral

Sources: leafly.com

Images: wsj.net bigbudsmag static1