Cannabis use has long been associated with various outcomes, ranging from income and legal problems to psychopathology. However, the existing understanding largely relies on correlational research designs that struggle to account for potential confounding variables. To address this limitation, a groundbreaking longitudinal study involving 4,078 American adult twins, assessed over several decades, employs cotwin control mixed effects models to explore the nuanced impact of lifetime average frequency of cannabis consumption on substance use, psychiatric, and psychosocial outcomes. This study titled “Limited psychological and social effects of lifetime cannabis use frequency: Evidence from a 30-year community study of 4,078 twins” was published in the Journal of Psychopathology and Clinical Science in January, 2024.
The study, with its unique twin-focused methodology, goes beyond traditional correlational approaches. By evaluating within-pair differences, which inherently control for shared genes and environments, the researchers aimed to uncover more nuanced insights into the causal relationships between cannabis use and various outcomes.
The average lifetime cannabis frequency among participants was one to two times per month throughout adolescence and adulthood. Initial individual-level analyses revealed the expected associations between cannabis use and a broad spectrum of outcomes. However, the within-pair comparisons unveiled intriguing insights. Three out of the 22 assessed outcomes demonstrated within-pair differences consistent with possible causality: cannabis use disorder symptoms, frequency of tobacco use, and illicit drug involvement.
The results suggest that more frequent cannabis use may cause small increases in cannabis use disorder symptoms, specifically around 1.3 symptoms when transitioning from once-a-year use to daily use. However, for other outcomes, familial confounding appears to play a more substantial role, at least within this community population of twins. The findings prompt a reevaluation of the assumed relationships between cannabis use and outcomes, emphasizing the importance of considering shared genetic and environmental factors in understanding the complex dynamics at play.
As debates surrounding cannabis legalization and its societal impact continue, this longitudinal twin study offers a unique perspective. By employing cotwin control models, the research adds granularity to our understanding of the associations between cannabis use and various life outcomes. It underscores the need for nuanced interpretations, considering the interplay of shared genetics and environments. These findings contribute valuable insights to ongoing discussions in both scientific and policy realms surrounding cannabis use and its consequences.