There is a growing body of data indicating that Gene x Child Maltreatment interactions at monoamine oxidase A (MAOA) play a role in vulnerability to symptoms of antisocial personality disorder (ASPD) but not major depression (MD). Using a sample of 538 participants from the Iowa Adoption Studies, we introduce a conceptual model that highlights two distinct pathways from child maltreatment to symptoms of MD, suggesting that maltreatment has different effects depending on genotype and highlighting the importance of including the indirect pathway through ASPD. As predicted by the model, high activity alleles predispose to symptoms of MD in the context of child maltreatment whereas low activity alleles predispose to symptoms of ASPD. We conclude that the Gene x Environment interplay at this locus (MAOA) contributes to both symptoms of ASPD and MD and that careful specification of child maltreatment may be essential if genetic association research is to produce replicable results.