Changes in the legalization of cannabis use have been widespread across the United States, accompanied by increased rates of use. This has led to a recent increase in concern that cannabis use has a negative impact on cognitive functioning. Most studies assessing this association have reported that persistent cannabis users who initiate use at earlier ages have a greater decline in IQ across time compared to those who occasionally use cannabis or initiate use at later ages. However, there are many potential confounders that may account for the association between cannabis use and decline in cognitive functioning (e.g., SES). Studies examining twin pairs who are discordant on the degree of cannabis use control for potential confounding caused by common genetic and family environmental factors that may influence both cannabis use and cognitive functioning. Such studies have consistently found that cannabis use does not cause cognitive decline.
The current study assessed within and between-families, in a sample of twins, the association between multiple measures of cannabis use and cognitive functioning. This study extends previous twin studies on this topic by using multiple measures of cannabis use (age of initiation and frequency of use in addition to disorder diagnosis) and factor scores of multiple executive functions (EFs; including a common factor and factors specific to updating working memory and shifting mental sets). Given the previous research results among twins, we hypothesized that cannabis use is not associated with individual differences in cognitive function. Analyses included about 750 individuals including about 380 twin pairs and about 10 individuals without co-twin data, depending on the models, from the Colorado Longitudinal Twin Study. Multilevel models were used to estimate the between-family and within-family associations between cannabis use (frequency, cannabis use disorder diagnosis, and age of initiation) and cognitive functioning (IQ and EFs) at ages 17 and 23, controlling for IQ measured prior to cannabis initiation (ages 7 to 12). Results indicated that cannabis use is significantly associated with measures of IQ and EF between families and within families. Specifically, between-families, twin pairs with younger age of initiation, greater frequency of cannabis use, and more severe cannabis use disorder diagnosis had lower IQ scores. Within-families, but not between-families, the twin with a greater frequency of cannabis use and more severe cannabis use disorder diagnosis had lower common EF. Next, we conducted the same analyses while also controlling for other substance use variables (i.e., alcohol, nicotine, and other drugs). The between-family and within-family cannabis associations were no longer significant or only marginally significant. Our results suggest that cannabis use is associated with concurrent IQ and EFs, even after controlling for IQ differences prior to cannabis initiation. However, most of these associations are not significant within families, suggesting familial confounding, and these between-family and within-family effects may be due to other substances that are used more frequently by individuals who use cannabis. These results are inconsistent with a simple causal model in which cannabis use causes declines in IQ or EF.
Cognition: Education, Intelligence, Memory, Attention , Substance use: Alcohol, Nicotine, Drugs
