This study utilized the Early Growth and Development Study (N = 561 adoptive children; 57.2% male, 55.3% White), a study of children adopted at birth, to examine heritable (birth parent psychopathology) and prenatal risk (prenatal maternal distress and smoking during pregnancy), infant negative affectivity, adoptive parent over-reactivity and warmth as independent predictors of childhood externalizing symptoms. The current study evaluated if: (1) infant negative affectivity and over-reactive parenting are candidate mediators for the effects of heritable and prenatal risk on externalizing symptoms and (2) parental warmth weakens the influence of heritable risk, prenatal risk, negative affectivity, and over-reactive parenting on externalizing symptoms. There were main effects of heritable risk, infant negative affectivity, and over-reactive parenting on child externalizing symptoms. The study found no support for the hypothesized mediation and moderation effects, suggesting that targeting parental over-reactivity rather than warmth would be more effective in reducing risk for childhood externalizing symptoms.