Short answer? No.
Researchers at UCLA and the University of Minnesota gave all stoners a reason to rest easy, after concluding that regular use of marijuana does not have an effect on IQ.
In the largest longitudinal twin study ever, researchers tested twins in six cognitive areas between 9-12 and then again at ages 17-20. Participants self-reported marijuana use and frequency, as well as genetic data, socioeconomic status, and family background.
The researchers were looking at three criteria: Does marijuana cause IQ decline, or is it merely associated with poor cognitive ability (meaning poor cognitive scores should only be evident after the study starts not before)? Does smoking more marijuana lead to a higher decrease in IQ? And finally, does marijuana usage and IQ decline continue to associate even after controlled for things like socioeconomic status and family background?
The researchers concluded that there was, “little evidence to suggest that adolescent marijuana use has a direct effect on intellectual decline.” They found no relationship between heavy marijuana use and long-term cognitive decline. In instances where one twin reported smoking and the other did not, differences completely disappeared when controlled for self-reported binge drinking and other substance abuse.
In one of their studies, researchers did find that IQ scores were initially lower in groups that would eventually go on to smoke marijuana, leading the researchers to conclude that other factors such as behavioral disinhibition and conduct disorder may be related to lower IQ and eventually, substance use.
A previous study coming out of Duke that concluded marijuana use did effect IQ has been widely criticized for failing to control for factors such as socioeconomic status and other environmental factors. And this new study is providing conclusive evidence if you toke up, your IQ points are safe and sound.
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