Leveraging parasympathetic nervous system activity to study risk for psychopathology: The special case of callous-unemotional traits

•Callous-unemotional (CU) traits predict particularly severe, stable, and harmful forms of antisocial behavior.•The parasympathetic nervous system (PNS) is a validated biomarker of psychopathology and reflects deficits in prefrontal cortex functioning.•We argue that measures of PNS functioning can significantly enhance our understanding of the etiology of CU traits.•We outline how disrupted PNS functioning can elucidate multiple etiological pathways to CU traits. Callous-unemotional (CU) traits are implicated in the development of severe and persistent antisocial behavior, and appear to arise from distinct behavioral, temperament, physiological, and neural risk factors to other types of externalizing psychopathology. Although existing etiological models have attempted to characterize specific mechanisms underlying the emergence of CU traits, prior empirical work is plagued by a host of inconsistent results. This inconsistency is particularly apparent in research that has investigated variation in physiological functioning, arousal, and reactivity. In this review, we summarize and evaluate studies that have examined the role of parasympathetic nervous system (PNS) functioning in child psychopathology, including studies of CU traits. We argue that the inclusion of measures of PNS activity can significantly enhance our understanding of the etiology of CU traits, provide clarity to the inconsistent findings of prior studies, and advance mechanistically-targeted interventions and treatments. We conclude the review by providing foundational and testable hypotheses about the role that disrupted PNS functioning could play in multiple pathways to CU traits beginning in infancy and across development.