Nonreciprocal action of the parasympathetic (PNS) and sympathetic (SNS) nervous systems, increases susceptibility to emotional and behavioral problems in children exposed to adversity. Little is known about the PNS and SNS in interaction with early adversity during infancy. Yet this is when the physiological systems involved in emotion regulation are emerging and presumably most responsive to environmental influences. We examined whether parasympathetic respiratory sinus arrhythmia (RSA) and sympathetic pre-ejection period (PEP) response and recovery at six months, moderate the association between cumulative prenatal risk and physical aggression at 20 months (N=113). Prenatal risk predicted physical aggression, but only in infants exhibiting coactivation of PNS and SNS (i.e., increase in RSA and decrease in PEP) in response to stress. These findings indicate that coactivation of the PNS and SNS in combination with prenatal risk is a biological marker for the development of aggression.