Twin Study Suggests Cannabis-Psychosis Link May Be Due to Shared Genetics | Research

The takeaway

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.

Psychiatrists, geneticists, cannabis-psychosis researchers

No evidence of causal effect when comparing twins who used different amounts of cannabis

What the researchers 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.

Why it 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 numbers in context

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 the study worked

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.

What this study cannot 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.

How to read the evidence

Strong genetically informative design using prospective twin cohort data, limited by dimensional rather than clinical outcome measure.

When this study was published

Longitudinal data with adolescent cannabis measures and adult psychoticism assessment.

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.

Questions still open

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?

Common 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.

Read the original research

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

View on PubMed ↗Publisher’s paper ↗

Citation

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