We’ve all seen the prototypical stoner teen, whether in real life or depicted in movies or on TV. Dull-witted, spaced out, these kids are the poster children for lost IQ points. The belief that marijuana causes a cognitive decline due to damaging the developing brain was reinforced by a 2012 study. Other studies have resulted in conflicting outcomes.
Two recent longitudinal twin studies negate the conclusion that marijuana causes neurocognitive damage to the developing brain.
One study followed 789 predominantly Hispanic kids from California from age 9-10 through age 19-20. During this 10-year period they were assessed 5 times.
The second study followed 2,227 predominantly non-Hispanic, Caucasian kids from Minnesota. These children were assessed at 3-year intervals
The kids were given IQ tests over the years and questioned about their marijuana usage. The results affirmed that the kids who smoked marijuana showed a greater decline in IQ over the years than the typical kid who did not. However, due to the fact that these were twin studies, the sets of twins where one smoked marijuana and the other did not revealed that marijuana was not the cause of intellectual decline.
When one twin who smoked marijuana showed a decline, so did the other, even when the other did not smoke marijuana. Since both of the twins showed a decline, lower test scores were attributed to their home environment and parental deficits such as “less parental monitoring” and “less emphasis on scholarship.”
These twin studies have once again proven that just because you can associate something (marijuana) with an outcome (cognitive decline), that doesn’t make it the cause.
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- Marijuana Use May Not Lead to Cognitive Decline After All – Pacific Standard Magazine
- Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies – Proceedings of the National Academy of Sciences of the United States of America