Hypothetical results expected from a discordant twin study, for discordant pairs of MZ, DZ and unrelated individuals, which can be used to infer the nature of the association between cannabis use and outcomes. If the association is entirely due to causal or individual-specific factors then the magnitude of association is invariant to degree of relatedness. In contrast, if genes or family environment play a role, then matching discordant pairs of individuals for these factors results in an attenuation of the association.