Dysregulated physiological stress reactivity has been suggested to impact the development of children and adolescents with important health consequences throughout the life span. Both environmental adversity and genetic predispositions can lead to physiological imbalances in stress systems, which in turn lead to developmental differences. We investigated genetic and environmental contributions to autonomic nervous system reactivity to a psychosocial stressor. Furthermore, we tested whether these effects were consistent with the differential susceptibility framework. Composite measures of adverse life events combined with socioeconomic status were constructed. Effects of these adversity scores in interaction with a polygenic score summarizing six genetic variants, which were hypothesized to work as susceptibility factors, were tested on autonomic nervous system measures as indexed by heart rate and heart rate variability. Results showed that carriers of more genetic variants and exposed to high adversity manifested enhanced heart rate variability reactivity to a psychosocial stressor compared to carriers of fewer genetic variants. Conversely, the stress procedure elicited a more moderate response in these individuals compared to carriers of fewer variants when adversity was low.