Thesis (Ph.D.) - Indiana University, Psychological and Brain Sciences, 2016Externalizing behavior problems including aggression, inattention/hyperactivity, oppositionality, and conduct problems are prevalent and burdensome. The present report describes two studies that examined neural mechanisms in the development of externalizing problems in very early childhood when self-regulation skills are rapidly developing and potentially most malleable. Study 1 (N = 27) and Study 2 (N = 64) examined neural functioning in relation to self-regulation and externalizing problems in 2 1/2- to 3 1/2-year-old children. Event-related potential (ERP) components and electroencephalography (EEG) assessed neurophysiological functioning during oddball and go/no-go tasks. A subset of the children were assessed with EEG measures at multiple occasions at 6 month intervals. Because deficient self-regulation is considered to reflect an underlying phenotype of externalizing behavior, we examined neurophysiological risk markers in relation to behavioral measures of self-regulation (inhibitory control and sustained attention) and questionnaire measures of externalizing behavior problems. Several neural markers were associated with self-regulation deficits and externalizing behavior problems: smaller oddball P3b amplitudes, longer oddball P3a latencies, too small or too large no-go N2 amplitudes, left frontal asymmetry, and less frontal alpha power. Less frontal alpha power, thought to reflect task-inefficient/excess neural processing, showed the greatest predictive value among these markers, and predicted relative increases in externalizing problems over time with moderate accuracy. Findings suggest that future studies should examine a new model of no-go N2 amplitudes in relation to inhibitory control whereby there may be an optimal range of inhibitory processing. Too little inhibitory processing may reflect poorer inhibitory control and too much may reflect the recruitment of excess/inefficient inhibitory resources. Future studies should examine how to ensure the validity of neurophysiological data while reducing lost EEG data due to artifacts and poor behavior performance in order to maximize generalizability and clinical utility. In sum, longitudinal studies of neural functioning in relation to behavior in early childhood have the potential to greatly advance our understanding of mechanisms in the development of self-regulation and behavior problems.