The response modulation hypothesis of psychopathy states that psychopaths' inability to adapt their behavior to changing circumstances results from an inability to process peripheral cues in the midst of a dominant response. The current study examined this relationship between psychopathic traits and response modulation using a lexical decision stop signal task and event-related potentials to assess deficits in stimulus and error processing. Overall, the impulsive antisocial (IA) factor of psychopathy was related to poor behavioral performance, particularly in relation to response inhibition. Additionally, IA was related to reduced processing of the stop signal itself as well as subsequent inhibition errors. This deficient processing of the stop signal was found to significantly mediate the relationship among IA, behavioral performance, and the processing of inhibition errors, indicating that a deficit in processing peripheral cues may be responsible for subsequent behavioral and error processing deficits in IA.