In a recent issue of PNAS, Meier et al. (1) used a prospective cohort of 1,037 New Zealanders followed from birth to age 38 y to show that the relationship between cannabis use and cognitive impairment cannot be attributed to precannabis-use differences in intelligence. However, important omitted “third” variables, such as stable individual differences in personality traits, could potentially cause both drug use and changes in intelligence. For instance, a negative relationship between cannabis use and neuropsychological performance may be explained by low conscientiousness, which could lead to substance dependence (2) and poor performance on neuropsychological tests (2, 3). Personality traits could also explain positive noncausal associations between cannabis use and cognitive functioning. For example, high levels of openness to experience could lead people to seek out activities that promote cognitive functioning and could also condition the initiation of cannabis use (4). It is, therefore, critical to consider whether personality traits may confound the cannabis–intelligence link.