This article argues in support of the proposition that “A Personality Disorder May Nullify Responsibility for a Criminal Act.” Building upon research in categorical and dimensional controversies in diagnosis, neurocognitive science and the behavioral genetics of mental disorders, and difficulties in differential diagnosis and co-morbidity with personality disorders, this article holds that a per se rule barring personality diagnosis as a basis for a defense of legal insanity is scientifically and conceptually indefensible. Rather, focus should be upon the severity and (...) impact in specific cases of any legally relevant functional deficits arising from a mental disorder (including personality disorders). Failure to do so risks potentially misleading “battles of the experts” about a defendant's diagnosis in criminal responsibility defenses and improper usurpation of the role of the legal finder of fact as mental health expert witnesses are inserted as gatekeepers indefensibly based upon diagnosis. Implications for practice and public policy are considered, including a “modest proposal” for post-trial management of defendants found not guilty by reason of insanity on the basis of functional deficits arising from personality disorder. (shrink)