Reduces IQ with marijuana

Recent studies raise doubts that smoking weed affects intelligence

Is smoking weed bad for your intelligence? Previous studies have found a link. However, current studies do not see smoking weed as a cause of unfavorable intelligence development. Rather, smoking weed and decreased intelligence are the result of other factors.

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In 2012, a study from New Zealand attracted attention. “Smoking weed makes you stupid” read the headlines in the media. When adolescents consume cannabis, this can cause their intelligence quotient (IQ) to decrease by up to 8 points. In two recent studies, however, there was no confirmation that cannabis is the cause of an IQ decline.

When asked whether behavior such as smoking weed has an impact on intelligence, researchers are usually faced with the problem that they have to rule out many other possible influencing factors. It is known that cannabis use is often associated with a number of other risk factors such as social problems in childhood or the use of other psychoactive substances. It cannot therefore be ruled out that adolescents who start smoking weed have had problems beforehand or consume other substances that also have an unfavorable effect on intelligence development.

A British research team led by Claire Mokrysz from University College London tried to take these methodological problems into account in a longitudinal study. The study examined over 2,000 adolescents who were accompanied from birth. Intelligence was measured for the first time at the age of 8. The measurement was repeated at the age of 15. In addition, the young people were asked whether and how often they had already consumed cannabis.

Tobacco smoking predicts IQ development

As in the study from New Zealand, there was initially a connection between the frequency of smoking weed and IQ at the age of 15. Adolescents who had smoked weed more than 50 times had an IQ 2.9 points lower than their peers who had never used cannabis. However, when the research team took into account other possible influencing factors such as problems in childhood or the use of other substances, there was no longer any significant connection between cannabis and IQ development.

Tobacco smoking had the greatest influence on IQ development. The research team also checked this connection on a sample of adolescents who smoked tobacco but had never smoked. It was shown here that smoking teenagers had an IQ 6.2 points lower than non-smoking teenagers. If other risk factors from childhood were included, the IQ was only 1.5 points lower, but remained significant.

However, Mokrysz and her team do not believe that tobacco smoking is a causal factor in a decrease in IQ. It is conceivable that young people with poor school grades are more likely to start smoking than good students. Smoking could also indicate other childhood problems. This also applies to cannabis use. Mokrysz and her team therefore emphasize that their study does not provide any evidence that cannabis contributes causally to unfavorable IQ development.

Study takes into account genes and the environment

This result is confirmed by a second study. Both the environment in which the young people grew up and their genes were taken into account. It is generally assumed that our genetic makeup plays a decisive role in the development of intelligence.

Study leader Nicholas Jackson and his team evaluated the data from two longitudinal studies that involved over 3,000 twins. Twins, especially identical ones, are genetically very similar. In addition, the twins grew up together, so they were mostly exposed to the same environmental influences in their childhood. When the first intelligence measurement was taken, the twins were between 9 and 12 years old. The tests were repeated between the ages of 17 and 20. Between the measurement times, some of the twins started smoking weed, others remained abstinent. There were also identical twins, one of whom smoked and the other did not.

No differences in twins

When comparing the IQ values ​​between the measurement times, the values ​​again showed poorer values ​​in the adolescents who had started smoking weed. The difference was up to four IQ points. However, the research team could not find any IQ differences in the twins, only one of whom had started smoking weed. The non-consuming twin also showed poorer IQ values ​​at the second measurement point in time. However, if cannabis were to have a detrimental effect on intelligence development, the abstinent twin should have done better than his brother or sister.

That means: It is not cannabis use, but another factor that both twins share that causes an IQ drop in them. The research team can only speculate about what exactly that is. Unlike the Norwegian researcher Ole Rogeberg, they do not assume that there are socio-economic differences. They had already taken socio-economic status into account in their analyzes.

Only crystalline intelligence affected

Jackson and his team, however, suspect that family-cultural deficits play a role. If parents generally do not have good contact with their children or care little about their children's educational success, then this could also have negative effects on intelligence development.

The assumption is supported by the fact that the IQ differences found relate exclusively to the so-called crystalline intelligence Respectively. The crystalline intelligence stands for general knowledge in the broadest sense, while the fluid intelligence An expression of the flexibility and speed of thinking is that it is independent of what has been learned. Jackson and his team argue that adolescents who are not as well supported in their educational path by their parents are less focused on a good education and therefore develop less developed crystalline intelligence.

It is also known, the research team explains, that adolescents who are not so good at school are also more likely to start using cannabis. Smoking weed in adolescence is therefore not the cause of a lower IQ, but, like crystalline intelligence, primarily a consequence of the parent-child relationship, argues the research team.

Smoking weed is still not safe for young people

The general all-clear that cannabis is safe for young people is still out of place, explains Professor George Patton from the University of Melbourne in the magazine of the renowned journal Science. He points out that the young people in Jackson and his team's study still consumed comparatively moderately. However, a number of studies have shown that heavy consumption in adolescence has been shown to have negative effects on brain development and cognitive abilities.

Jackson doesn't want to give smoking weed either. The study's findings “do not mean that heavy cannabis use is unproblematic among young people,” he explains. Other important everyday skills can also be affected by smoking weed. Jackson sees the problems primarily in the parent-child relationship: "I worry most about what goes on in the environment of a child when he or she takes refuge in drugs at the age of 14."


What influence cannabis has on intelligence development remains controversial in science. Two recent studies raise doubts that smoking weed is responsible for lower intelligence in adolescence. However, both studies cannot completely eliminate the possibility that heavy cannabis use has a negative effect on intelligence development. Other studies suggest that early entry and regular consumption in adolescence have a lasting effect on brain development and cognitive abilities.

However, both studies have also made it clear that the one-sided focus on substance use may cause other important factors to be disregarded. Because the quality of the parent-child relationship seems to have a significant share not only on the development of intelligence, but also on the substance use of the children.


  • Jackson, N. J., Isen, J. D., Khoddam, R., Irons, D., Tuvblad, C., Iacono, W. G., McGue, M., Raine, A. & Baker, L. A. (2016). Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies. PNAS, doi: 10.1073 / pnas.1516648113.
  • Mokrysz, C., Landy, R., Gage, S. H., Munafò, M. R., Roiser, J. P. & Curran, H. V. (2016). Are IQ and educational outcomes in teenagers related to their cannabis use? A prospective cohort study. Journal of Psychopharmacology, 30 (2), 159-168.
  • Sciencemag (January 18, 2016)