There has been much debate as to whether the association between cannabis and subclinical expression of psychosis is causal, or whether psychotic experiences may prompt cannabis use in individuals at genetic risk for psychosis as a means of self-medication. The Genetic Risk and Outcome in Psychosis (GROUP) study investigated the association between familial liability for psychosis and sensitivity to cannabis, using patient sibling and sibling-control pairs analyses. This study focused on gene-environment interactions relevant to psychotic disorders, and included patients with psychotic disorder, their siblings and community controls. Caspi and colleagues highlighted the importance of individual genetic vulnerability when they reported an interaction between cannabis use and variation in the gene that encodes catecholamine-O-methyl transferase (COMT). Given that different types of cannabis clearly affect mental health differentially, more research is needed to understand how genetic liability may increase sensitivity to, or preference for, the specific constituents of cannabis.