The determinative issue in applying the insanity defense is whether the defendant experienced a legally relevant functional impairment at the time of the offense. Categorical exclusion of personality disorders from the definition of mental disease is clinically and morally arbitrary because it may lead to unfair conviction of a defendant with a personality disorder who actually experienced severe, legally relevant impairments at the time of the crime. There is no need to consider such a drastic approach in most states and in the federal courts, where the sole test of insanity is whether the defendant was - unable to appreciate the wrongfulness of his conduct at the time of the offense. This is because the only symptoms that are legally relevant in such jurisdictions are those that impair reality-testing and thereby affect the person’s capacity to understand the nature and consequences of her actions. However, if the test of insanity includes a - volitional prong‖ (inability to control one’s behavior), some way must be found to limit the scope of the defense to the core cases (involving psychotic conditions) to which it has traditionally been applied, and to prevent a shift toward a deterministic account of criminal conduct - i.e., "people can't help being who they are and doing what they do." The best way of accomplishing this is to limit the definition of mental disease to severe disorders characterized by gross disturbances of the person’s capacity to understand reality.
Richard J. Bonnie, Should a Personality Disorder Qualify as a Mental Disease in Insanity Adjudication?, 38 Journal of Law, Medicine, & Ethics 760–763 (2010).