Does smoking lots of pot make you dumb or do dumb people smoke lots of pot? Mostly, the latter. Ross et al. (2019) write:
Although many researchers have concluded that cannabis causes impairment in cognition, there are alternative explanations. First, poor cognitive functioning is a risk factor for substance use. Specifically, EF measured in childhood predicts later substance use and substance use disorders (SUDs; Ridenour et al., 2009). Thus, studies need to control for prior cognitive functioning (Meier et al., 2012). Second, poor cognitive functioning and cannabis use may also be related, not because one causes the other, but because they share common risk factors, like lower SES (Rogeberg, 2013). Lynskey and Hall (2000) proposed that early use is likely to occur in a social context characterized by affiliations with substance using peers, poor school attendance, and precocious adoption of adult roles including dropping out of school; such an effect on educational participation may also influence later cognitive functioning.
Indeed–twin studies which control for genetics and family environment–do not find that cannabis reduces cognition:
Lyons et al. (2004) examined MZ twins discordant for use 20 years after regular use, and found a significant difference between twins on only one of 50+ measures of cognition. Second, Jackson et al. (2014) found no evidence for a dose-dependent relationship or significant differences in cognition among MZ twins discordant for cannabis use. Similarly, Meier et al. (2017) found no evidence for differences in cognition among a combined sample of MZ and DZ twins discordant for cannabis dependence or use frequency. Thus, quasi-experimental, co-twin control designs have yielded little evidence that cannabis causes poorer cognition.
Ross et al. run a similar study but testing also for executive function skills–the ability to plan, focus, control impulses and so forth which are skills related to IQ but distinct–and they conclude:
Families with greater cannabis use showed poorer general cognitive ability. Yet within families, twins with higher use rarely had lower cognitive scores. Overall, there was little evidence for causal effect of cannabis on cognition.
Hat tip: The excellent Kevin Lewis.