Children referred for externalizing behavior problems may not represent a homogeneous population. Our objective was to assess neural mechanisms of emotion regulation that might distinguish subtypes of externalizing children from each other and from their normal age mates. Children with pure externalizing~EXT! problems were compared with children comorbid for externalizing and internalizing~MIXED! problems and with age-matched controls. Only boys were included in the analysis because so few

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... were referred for treatment. We used a go0no-go task with a negative emotion induction, and we examined dense-array EEG data together with behavioral measures of performance. We investigated two event-related potential~ERP! components tapping inhibitory control or self-monitoring-the inhibitory N2 and error-related negativity~ERN!-and we constructed source models estimating their cortical generators. The MIXED children's N2s increased in response to the emotion induction, resulting in greater amplitudes than EXT children in the following trial block. ERN amplitudes were greatest for control children and smallest for EXT children with MIXED children in between, but only prior to the emotion induction. These results were paralleled by behavioral differences in response time and performance monitoring. ERP activity was localized to cortical sources suggestive of the dorsal anterior cingulate for control children, posterior cingulate areas for the EXT children, and both posterior cingulate and ventral cingulate0prefrontal regions for the MIXED children. These findings highlight different mechanisms of self-regulation underlying externalizing subtypes and point toward distinct developmental pathways and treatment strategies.