Abnormal patterns of sympathetic- and parasympathetic-linked cardiac activity and reactivity are observed among externalizing children and mark deficiencies in central nervous system regulation of behavior and emotion. Although changes in these biomarkers have been observed following treatment, mechanisms remain unexplored. We used MEMORE—a new approach to analyzing intervening variable effects—to evaluate improvements in parenting as mediators of changes in sympathetic nervous system (SNS)- and parasympathetic nervous system (PNS)-linked cardiac activity and reactivity among 99 preschoolers with attention-deficit hyperactivity disorder who were treated using an empirically supported intervention. Decreases in negative parenting (criticism, negative commands, physical intrusions) were associated with increases in resting respiratory sinus arrhythmia (RSA) and pre-ejection period (PEP) reactivity to incentives from pre- to postintervention. Increases in positive parenting were not associated with changes in autonomic function. These findings suggest socially induced plasticity in peripheral biomarkers of behavior and emotion regulation and underscore the importance of reducing aversive interactions between parents and children when treating externalizing behavior.