Twin Study Suggests Cannabis-Psychosis Link May Be Due to Shared Genetics
While adolescent cannabis use was associated with higher adult psychoticism scores, co-twin control analyses found no evidence of a causal effect, suggesting shared family factors explain the link.
Quick Facts
What This Study Found
In 1,544 twins, both cumulative adolescent cannabis use and cannabis use disorder were associated with higher adult Psychoticism scores, but comparing twins within pairs (where one used more cannabis) showed no within-pair difference, pointing to familial confounds rather than causal effects.
Key Numbers
1,544 twins for cannabis use analysis; 1,458 for CUD analysis; no significant within-pair effect in co-twin models; no interaction between polygenic risk for schizophrenia and cannabis use on psychoticism.
How They Did This
Longitudinal co-twin control analysis using two cohorts of twins with prospective measures of adolescent cannabis use (N=1,544) and cannabis use disorder symptoms (N=1,458) linked to adult Psychoticism (PID-5), with polygenic risk scores for schizophrenia.
Why This Research Matters
This twin design is one of the strongest methods for separating causal effects from confounding. Finding no within-twin-pair effect challenges the assumption that cannabis directly causes psychotic traits and suggests shared genetic or environmental factors drive both.
The Bigger Picture
If the cannabis-psychosis link is largely explained by shared genetic liability rather than direct causation, it would shift the focus of prevention from cannabis avoidance toward identifying and supporting individuals with underlying genetic vulnerability.
What This Study Doesn't Tell Us
Psychoticism scale measures a broad dimension, not clinical psychosis diagnosis; predominantly White sample from Minnesota; may not generalize to populations with different genetic backgrounds; cannabis potency not measured.
Questions This Raises
- ?Would similar co-twin designs find the same result using clinical psychosis outcomes rather than dimensional psychoticism?
- ?Does cannabis potency matter more than frequency in this relationship?
Trust & Context
- Key Stat:
- No evidence of causal effect when comparing twins who used different amounts of cannabis
- Evidence Grade:
- Strong genetically informative design using prospective twin cohort data, limited by dimensional rather than clinical outcome measure.
- Study Age:
- Longitudinal data with adolescent cannabis measures and adult psychoticism assessment.
- Original Title:
- Adolescent cannabis use and adult psychoticism: A longitudinal co-twin control analysis using data from two cohorts.
- Published In:
- Journal of abnormal psychology, 130(7), 691-701 (2021)
- Authors:
- Schaefer, Jonathan D(2), Jang, Seon-Kyeong, Vrieze, Scott(4), Iacono, William G, McGue, Matt, Wilson, Sylia
- Database ID:
- RTHC-03500
Evidence Hierarchy
Follows a group of people over time to track how outcomes develop.
What do these levels mean? →Frequently Asked Questions
Does this study prove cannabis doesn't cause psychosis?
Not definitively. It found that the association between adolescent cannabis use and adult psychotic traits was explained by shared family factors. However, it measured broad psychoticism traits rather than clinical psychosis diagnoses.
What is a co-twin control design?
It compares twins within the same pair where one twin used more cannabis than the other. Since twins share genetics and family environment, any remaining difference can be more confidently attributed to cannabis itself. This study found no such difference.
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Cite This Study
https://rethinkthc.com/research/RTHC-03500APA
Schaefer, Jonathan D; Jang, Seon-Kyeong; Vrieze, Scott; Iacono, William G; McGue, Matt; Wilson, Sylia. (2021). Adolescent cannabis use and adult psychoticism: A longitudinal co-twin control analysis using data from two cohorts.. Journal of abnormal psychology, 130(7), 691-701. https://doi.org/10.1037/abn0000701
MLA
Schaefer, Jonathan D, et al. "Adolescent cannabis use and adult psychoticism: A longitudinal co-twin control analysis using data from two cohorts.." Journal of abnormal psychology, 2021. https://doi.org/10.1037/abn0000701
RethinkTHC
RethinkTHC Research Database. "Adolescent cannabis use and adult psychoticism: A longitudin..." RTHC-03500. Retrieved from https://rethinkthc.com/research/schaefer-2021-adolescent-cannabis-use-and
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.