Does cannabis lower IQ?

The way cannabis affects cognitive abilities, and in particular its impact on IQ, is often cited as an argument to bring up the adverse side effects of cannabis. Between the "yes" camp and the "no" camp, who is right?

Yes, cannabis lowers IQ

Most of the arguments in favor of a decline in IQ are based on a single study, the’Dunedin Study conducted by Madeline Meier. Meier followed 1,037 people born between 1972 and 1973 in Dunedin, New Zealand, from birth until age 38—a sample representing 91% of the population born during that period.

Like all studies, this one has its limitations. However, the authors have candidly acknowledged these limitations and have attempted to minimize their impact by excluding many of the most common confounding factors that could provide alternative explanations for the decline in IQ, including:

• Tobacco addiction
• Addiction to hard drugs (e.g., heroin, cocaine, amphetamines)
• Alcohol Dependence
• Schizophrenia

The authors examined six hypotheses:

• Everyday Cognition: Does cannabis-induced neuropsychological impairment lead to functional problems in daily life? The authors argue that it does
• Cognitive Decline: Long-term cannabis users appear to experience a greater decline in cognitive performance from childhood through adulthood than non-users
• Education: Some evidence suggests that staying in school can boost intelligence. Could long-term cannabis users experience neuropsychological decline simply because they dropped out of school to pursue other opportunities?
• Recovery Hypothesis: Long-term users who quit or reduce their cannabis use may be able to restore their neuropsychological health
• Developmental Vulnerability: Because cannabis has increased toxic effects on the developing brain, adolescents are particularly vulnerable to the effects of persistent cannabis use
• Specificity: Are the impairments limited to specific neuropsychological domains, or are they present in each of the five domains mentioned above? The authors hypothesized that the impairments are not limited to specific cognitive domains

What were the conclusions?

The study’s findings were as follows:

• Early exposure to cannabis is associated with a lower IQ in adulthood, and more persistent use is associated with a greater decline in IQ
• No decrease in IQ was observed among individuals who waited until adulthood to use cannabis. This finding was consistent not only among occasional users but also among those with a substance use disorder.

What is the difference between consumers and non-consumers?

The study participants had their IQ tested five times, at ages 18, 21, 26, 32, and 38.

Abstainers: Of the 1,037 study participants, 242 reported never having used cannabis. These participants experienced a slight increase in their IQ, from 99.84 to 100.64.

Occasional consumers: 479 participants reported having used cannabis at some point but had never been diagnosed with a substance use disorder. This group experienced a modest decline in IQ, from 102.32 to 101.25—approximately 1 point.

Problem consumers: Among the 38 participants who met the criteria for cannabis use disorder at three of the five follow-up assessments, the decline in IQ was much more pronounced, with a drop of nearly 6 points, from 99.68 to 93.93.

One particularly interesting finding is that the average IQ of occasional users was 2.5 points higher than that of the abstinent group before the introduction of cannabis. By age 38, the IQ gap had narrowed, but the group of occasional users still maintained an IQ advantage of 0.61 points. Does this mean that abstainers are less intelligent than occasional users?

No, cannabis does not lower IQ

The initial findings support the critics of the Dunedin study. Despite the study’s high quality, several scholars have questioned the factors that might explain the decline in IQ.

The sample of problem consumers is too small. With only 38 heavy cannabis users, the small sample size limits the generalizability of the findings. Meier’s study, however, included three other subgroups of early users, all of whom experienced a decline in IQ proportional to the duration of their use.

Certain personality traits explain the decline in IQ. British scientist Michael Daly, a behavioral specialist, suggested that personality traits might explain positive non-causal associations between cannabis use and cognitive functioning. A high level of experience seeking could lead people to seek out activities—including experimenting with cannabis—that promote cognitive functioning. Daly analyzed data from 6,401 participants in the British study. His post-hoc analysis revealed that experience-seeking positively influenced cannabis use and improved neuropsychological functioning.

Socioeconomic factors account for the decline in IQ, as proposes it Norwegian economist Ole Rogeberg. His argument is based on the fact that cannabis use is more prevalent among the less privileged classes. Since attending school temporarily boosts IQ, it drops again later on. However, socioeconomic status is a poor predictor of cannabis use, which affects all social classes without distinction. Moreover, Meier’s study did not identify a decline in IQ between adolescence and adulthood among individuals from disadvantaged socioeconomic backgrounds.

There is also a British study involving 2,235 students which concluded that «short-term cannabis use during adolescence does not appear to lead to a decrease in IQ or impaired brain function, even when cannabis use reaches the level of dependence.».

A Study of IQ in Twins

A good way to eliminate as many factors as possible that might influence the results is probably to conduct a test on twins.

Actually, a study titled «Impact of Adolescent Cannabis Use on Intelligence: Results from Two Longitudinal Twin Studies» compared changes in IQ among identical twins who used cannabis or abstained from it over a 10-year period. After accounting for family factors that could influence the results, the scientists found no link between cannabis use and a decline in IQ. In groups of twins where one twin used cannabis and the other did not, comparable declines in IQ were observed, supporting the hypothesis that other family-related factors, such as genetics or the environment, may explain lower IQ test scores.

The study also has limitations, including a lack of detail regarding the intensity of consumption and differences in the questions used to gather data between the two populations studied (one in Los Angeles and the other in Minnesota)

Who's right?

All we can say based on these studies is that, for adults, cannabis use does not appear to affect IQ. However, regular use is associated with adverse cognitive effects that appear to be reversible. For adolescents, the impact of cannabis use has not yet been determined, bearing in mind that the brain is particularly vulnerable during its development.