Cannabis and Psychotic Experiences in Teens Are Linked by Shared Environment, Not Shared Genes | Research

The takeaway

In nearly 10,000 sixteen-year-old twins, the association between cannabis use and psychotic experiences was almost entirely explained by shared environmental factors rather than shared genetic vulnerability.

Read this if you are interested in whether the cannabis-psychosis link is genetic or environmental.

69-100% of the cannabis-psychosis association due to shared environment

What the researchers found

Researchers used data from 4,830 twin pairs (aged 16) to determine whether the cannabis-psychosis association is driven by genetics, shared environment, or unique environment.

Cannabis use was modestly heritable (37%) with strong shared environmental influence (55%). Psychotic experiences had variable heritability (27-54%) depending on the specific symptom type.

The key finding: environmental influences explained virtually all of the covariation between cannabis use and paranoia, cognitive disorganization, and negative symptoms (69-100% from shared environment). Only the relationship between cannabis and hallucinations showed some familial (possibly genetic) influence.

Cannabis use explained only 2-5% of the total variance in psychotic experiences, suggesting the effect, while real, is modest.

Why it matters

If cannabis and psychotic experiences co-occur primarily because of shared environments (rather than shared genes), this suggests that modifying environmental factors (neighborhood, peer groups, family context) could reduce both cannabis use and psychotic risk simultaneously.

The numbers in context

4,830 twin pairs; cannabis heritability 37%; shared environment 55%; cannabis explains 2-5% of variance in psychotic experiences; 69-100% of covariation due to shared environment

How the study worked

Twin study of 4,830 16-year-old pairs using structural equation modeling. Self-reported and parent-reported psychotic experiences. Self-reported cannabis use. Multivariate liability threshold models estimated genetic and environmental contributions to covariation.

What this study cannot tell us

Cross-sectional design in 16-year-olds. Twin modeling assumptions may not perfectly hold. Self-reported cannabis use at 16 may underestimate true use. Cannot identify which specific environmental factors drive the association.

How to read the evidence

Large twin study with sophisticated modeling, providing strong evidence for environmental mediation. Cross-sectional design limits causal conclusions.

When this study was published

Published in 2015. The cannabis-psychosis relationship continues to be studied with genetics and environmental methods.

The bigger picture

The finding that cannabis and psychotic experiences "travel together" due to environmental factors shifts focus from genetics to modifiable risk factors. Understanding which specific environmental factors drive both could enable more effective prevention.

Questions still open

Which specific environmental factors drive both cannabis use and psychotic experiences? Would environmental interventions that reduce cannabis access also reduce psychotic experiences? Does the genetic link with hallucinations specifically suggest a distinct biological pathway?

Common questions

Does cannabis cause psychosis because of genetics?

This study found the opposite: the association between cannabis use and psychotic experiences was almost entirely explained by shared environmental factors, not shared genetics. This means the environments that promote cannabis use also promote psychotic experiences.

How much does cannabis contribute to psychotic experiences?

Cannabis use explained only 2-5% of the total variance in psychotic experiences in this teenage sample. While statistically significant, this is a modest contribution, suggesting many other factors are more important.

Read the original research

Psychotic experiences are linked to cannabis use in adolescents in the community because of common underlying environmental risk factors.

Psychiatry research, 227(2-3), 144-51

View on PubMed ↗Publisher’s paper ↗

Citation

Shakoor, Sania; Zavos, Helena M S; McGuire, Philip; Cardno, Alastair G; Freeman, Daniel; Ronald, Angelica. (2015). Psychotic experiences are linked to cannabis use in adolescents in the community because of common underlying environmental risk factors.. Psychiatry research, 227(2-3), 144-51. https://doi.org/10.1016/j.psychres.2015.03.041