BACKGROUND: Early childhood aggressive behaviour is a predictor of future violence. Therefore, identifying risk factors for children's aggressive behaviour is important in understanding underlying mechanisms. Maternal postpartum depression is a known risk factor. However, little research has focused on the influence of paternal behaviour on early childhood aggression and its interaction with maternal postpartum depression. METHODS: This study was performed in two cohorts: the Fathers Project, in the United Kingdom (n = 143) and the Generation R Study, in The Netherlands (n = 549). In both cohorts, we related paternal antisocial personality (ASP) traits and maternal postpartum depressive (PPD) symptoms to childhood aggressive behaviour at age two (Fathers Project) and age three (Generation R Study). We additionally tested whether the presence of paternal ASP traits increased the association between maternal PPD-symptoms and early childhood aggression. RESULTS: The association between paternal ASP traits and early childhood aggressive behaviour, corrected for maternal PPD-symptoms, was similar in magnitude between the cohorts (Fathers Project: standardized β = 0.12, p = 0.146; Generation R: β = 0.14, p = 0.001), although the association was not statistically significant in the Fathers Project. Strikingly, and in contrast to our expectations, there was evidence of a negative interaction between paternal ASP traits and maternal PPD-symptoms on childhood aggressive behaviour (Fathers Project: β = -0.20, p = 0.020; Generation R: β = -0.09, p = 0.043) in both studies. This meant that with higher levels of paternal ASP traits the association between maternal PPD-symptoms and childhood aggressive behaviour was less and vice versa. CONCLUSIONS: Our findings stress the importance of including both maternal and paternal psychopathology in future studies and interventions focusing on early childhood aggressive behaviour. ; Wellcome Trust