Genetic studies have shown increased criminality in natural children of criminal fathers and an extra Y chromosome in many aggressive males. Biochemical studies have found low serum cholesterol in children with attention deficit disorder, a positive correlation between elevated plasma testosterone and aggression, and higher violent criminality by females during the premenstrual week. Physiological studies have focused on the connections between autonomic reactivity, avoidance learning, and need for stimulation in connection with aggression. Hyperactivity has been found to be associated with conduct disorder in children, and delinquency occurs more often in adolescents with histories of hyperactivity. Learning problems and lower IQ scores are also significantly associated with juvenile delinquency and violent behavior, even in studies controlling for race and class. Neurological disease may also play a part in delinquent behavior. Finally, recent research is focusing on the links between violent behavior with physiological and pharmacological abnormalities within the central nervous system. The findings indicate the need for clear neuropsychiatric assessments of organic problems before undertaking biological treatment of conduct-disordered adolescents. However, clinical experience has suggested that several medications affect aggressiveness, although noncompliance and rejection complicate efforts to treat adolescents with medications. 47 references.