Twin Study Finds Cannabis Legalization Did Not Cause Substantial Psychological Harm in Adults Interview with:
Stephanie Zellers PhD
Postdoctoral Researcher, Kaprio Group, FIMM
Dr. Zellers began this research as a graduate student at the
University of Colorado Boulder’s Institute for Behavioral Genetics (IBG)

Dr. Zellers PhD What is the background for this study?

Response: Proponents and opponents of cannabis legalization respectively cite various potential benefits and harms that these policies may cause. Many studies have evaluated these outcomes, but drawing causal conclusions is challenging due to the many confounds that may better explain observed effects.

 We investigated the effects of cannabis legalization on a broad range of psychological outcomes, like substance use, psychiatric symptoms, general functioning in daily life, and cognitive ability. We used a longitudinal twin sample with twin pairs living in different types of states (recreationally legal vs. recreationally illegal) to draw causally informative conclusions. Because identical twins share 100% of their genes, as well as environmental factors like the family rearing, SES, and community norms, co-twin control studies can rule out many alternative explanations for an observed relationship.