The best-established and replicated G×Es for psychological disorders are the interaction between a functional variable number tandem repeat in the monoamine-oxidase-A gene (MAOA) and childhood maltreatment in the onset of antisocial and violent behaviour [21,22] and the interaction between the serotonin transporter gene functional length polymorphism (5HTTLPR) and childhood maltreatment in the etiology of depression [23,24]. This evidence concerns the gene MAOA and major depressive disorder.