This study examined P3 event-related brain potential amplitude and the age of onset of adolescent problem behaviors associated with the development of externalizing psychopathology. Five hundred and one male and 627 female 17-year-old twins reported whether and when they had initiated tobacco, alcohol, or illicit drug use, had police contact, or had sexual intercourse. P3 amplitude was recorded using a visual oddball task. Each of these behaviors was associated with reduced P3 amplitude. When these five behaviors were used to create a composite early problem behavior scale reflecting onset prior to age 15, higher scores were associated with smaller P3 amplitudes. P3 amplitude reduction has been associated with genetic risk for alcoholism and other externalizing disorders associated with disinhibited behavior. Our results suggest that reduced P3 may also be associated with early expression of behaviors that predict the development of these disorders.