Study moderators examined interactions between children’s parasympathetic and sympathetic nervous systems (PNS and SNS) activity in order to achieve a greater specificity in the prediction of externalizing problems in the context of interparental conflict. Findings are robust across the three studies and provide the first reported evidence of interactions between PNS and SNS activity. Findings extend current theory indicating the importance of multisystem investigations for clarifying inconsistencies and discrepancies in the literature linking environmental stress, physiological responses, and child adjustment.