As children, before their first joint, marijuana smokers had IQ scores close to average.
However, by the time the individuals studied reached age 40, the IQ scores of pot smokers dropped significantly compared to their peers.
Those who were consistently dependant on marijuana during adulthood had the largest drop at 5 points. This drop may not sound like much, but it dramatically affect where one ranks among the general population’s IQ. An average kid with an IQ of 100 will be by definition smarter than 50% of the population. With a 5 point drop in their IQ from marijuana, they would only be smarter than 37% of the population by the time they are an adult.
Likewise, a bright young child in the 63rd percentile would end up barely average after a 5 point IQ decline.
It also matters when you start using marijuana. Heavy smokers who started before they turned 18 had an 8 point drop in their IQ, the largest IQ drop of all groups studied, while those who started as adults had a barely detectable 2 point drop.
Going back to the IQ distribution, we can see that if an average guy started smoking pot as a kid, they would have fallen to the bottom 30% of intelligence by the time they reached adulthood, a significant drop:
In fact, adults who start using marijuana as kids but stopped regular use as adults, still experienced a significant drop of 4 IQ points. This suggests that even some exposure to regular marijuana use as a kid is enough to cause life long changes in intelligence.
One possible explanation is that kids are particularly susceptible to any negative from marijuana due to the fact that their brains are undergoing rapid development.
It’s also possible that marijuana may affect learning in school. Much of IQ is determined by learning, schooling, even habits. If students were to experience a behavior change that affects their learning in high school, it’s possible that they may not receive the same IQ boosting education that they would have without marijuana.
When we dive deeper into the data, we see that not all forms of intelligence are affected equally. Most of the negative effects of marijuana are concentrated in verbal IQ and vocabulary.
Meanwhile, math and visuospatial, two other key components of IQ, are not significantly affected at all.
This suggests that contrary to what many mothers may say, marijuana does not appear to “kill your brain cells,” or cause a universal loss in intelligence. Rather, it affects very specific types of intelligence that can be explained by behavior changes.
For example, if marijuana smokers don’t study as many vocabulary flash cards for the SAT as their non-smoking classmates, we would expect to see this same pattern.
This theory is further supported by a recent study examining identical twins where one used marijuana and one did not. This type of study is especially powerful since we control for genetics, home environment, and almost everything else except for marijuana smoking.
As we can see above, even among twins, the deficiencies in vocabulary persist, despite the fact that IQ differences were found to be insignificant.
Based on the data, its pretty clear that IQ is significantly affected by smoking marijuana. Since the decline in IQ is specific to certain types of IQ, namely verbal IQ and vocabulary, the effect of marijuana may be driven by behavior rather than brain damage.
However, it remains unclear is whether this is caused by marijuana, or just correlated with marijuana use. For example, its entirely possible that personality traits that lend themselves to using marijuana also predispose people to have lower IQs later in life.
1) Meier, Madeline H., et al. "Persistent cannabis users show neuropsychological decline from childhood to midlife." Proceedings of the National Academy of Sciences (2012): 201206820.
2) Jackson, Nicholas J., et al. "Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies." Proceedings of the National Academy of Sciences 113.5 (2016): E500-E508.