Our results indicate a model of interaction known as ‘qualitative G×E interaction’ with a crossover pattern: carriers of risk allele(s) for one of the two genes (DRD2 rs1076560 T or AKT1 rs2494732 C allele), compared with individuals carrying no ‘risk’ alleles (DRD2 rs1076560 GG/AKT1 rs2494732 TT), have a lower probability of psychotic disorder if they never used cannabis but a higher probability if they have a history of cannabis use, especially of daily use. The gene discussed is AKT1; the disease is psychotic disorder.