The M'Naghten rule, which introduced the concept of the insanity plea in 1843, was based on the idea that certain people possessed a mental defect that made them unable to apprehend the consequences of their actions. The Durham rule attempted to add the notion of disease or mental infirmity to the legal concept of insanity. Finally, the legal definition of the American Law Institute (ALI) asked whether the defendant lacked substantial appreciation of his behavior and capacity to conform to legal requirements. This evolution has progressed in parallel to neuropsychological discoveries regarding various brain functions. An examination of Jeffrey Dahmer, an accused (and later convicted) serial killer revealed that he suffered from a severe personality disorder, characterized by antisocial, obsessive-compulsive, sadistic, fetishistic, and necrophilic features. His emotional conflicts were embedded in his personality structure and were not amenable to common psychotherapeutic or pharmacologic intervention. The jury convicted the defendant, convinced that he was legally sane when he committed the crimes of which he stood accused. 1 note and 19 references