Summary : Psychopathy and Antisocial Personality Disorder (ASPD) are important syndromes associated with persistent antisocial behavior. However, the asymmetric overlap between them creates an obstacle to understanding the mechanisms that differentiate them. To circumvent this difficulty, we have begun conducting studies comparing the criminal conduct and laboratory performance of offenders: 1) who meet diagnostic criteria for both ASPD and psychopathy, 2) who meet diagnostic criteria for ASPD but not psychopathy, and 3) who have neither ASPD nor psychopathic traits. These studies provide evidence that affective deficits, left hemisphere activation deficits, and specific indices of affect dysregulation are unique to psychopathy. Additional studies address the extent to which these mechanisms account for relationships between psychopathy and criminal conduct.