While 12% of adults in the US claim to have smoked cannabis, the same figure stands at 22% for those aged between 18 and 29. Yet, despite the substance’s increasing popularity, research has been limited on how it may affect intelligence, with some proposing that it may have long term negative effects. Now, researchers have found that this may not be the case, and that using cannabis does not necessarily lower IQ.
To reach this conclusion, researcher Nicholas Jackson from the University of Southern California teamed up with William Iacono from the University of Minnesota to examine data from two longitudinal studies of adolescent twins in California and Minnesota. During the study, the researchers measured the twins’ IQ between ages nine and 12 before drug use, and then at ages 17 and 20.
When analyzing their results, they found that cannabis use and IQ were uncorrelated, and that there was little to no difference in IQ scores between users and abstainers. Twin studies thereafter, including one performed in the UK, went on to confirm their findings. Jackson and Iacono did however find that cannabis users tended to have lower IQ test scores to begin with, and had significant reductions in IQ over time.
What explains these results? Firstly, young cannabis users are also more likely to use alcohol and other illicit drugs in adolescence than non-users. When the epidemiologists factored in binge drinking and other drug use in their models, they found that declines in IQ seemed to be more related to general substance use than cannabis alone.
The researchers then found other vulnerability factors that explained cannabis use and lower IQ scores. For example, they found that behaviors such as high impulsivity and excessive risk-taking were predictive of both substance abuse and having a lower IQ. They also found that being raised in a family that did not put much emphasis on education was a contributing factor.