Extreme and repeated aggression with little or no provocation occurs in a very small percentage of the population and appears to have neurological and possibly genetic components. Such behavior may originate in the limbic system of the brain, which is an important anatomical and physiological substrate for emotion and visceral activity. There also appears to be a correlation between extreme or repeated aggression and neurological abnormalities, including brain damage and epilepsy, although the abnormality does not necessarily imply the presence of violent behavior. Symptoms such as paranoia and loose associations also correlate strongly with violence, and the use of alcohol and other drugs exacerbates violence in the predisposed. Childhood abuse and observed abuse are the most important environmental factors associated with aggression, but socioeconomic level per se does not appear to be an important factor. Violence is difficult to treat, and neither drug therapy nor psychotherapy appears to prevent recidivism in all cases. While the preponderance of evidence does indicate that extreme violence as a behavioral syndrome is organically determined, protection of the public suggests that the use of the insanity defense for repeatedly violent offenders should be changed. 79 references. (Author abstract modified)