In its narrowest sense, "mentally disordered offender" refers to the approximately twenty thousand persons per year in the United States who are institutionalized as not guilty by reason of insanity, incompetent to stand trial, and mentally disordered sex offenders, as well as those prisoners transferred to mental hospitals. The real importance of mentally disordered offenders, however, may not lie in this figure. Rather, it may reside in the symbolic role that mentally disordered offenders play for the rest of the legal system. The 3,140 persons residing in state institutions on an average day in 1978 as not guilty by reason of insanity (see Chapter 4), for example, are surely worthy of concern in their own right. But they represent only 1% of the 307,276 persons residing in state and federal prisons in the same period (U. S. Dept. of Justice, 1981). From a purely numeric point of view, the insanity defense truly is "much ado about little" (Pasewark & Pasewark, 1982). The central importance of understanding these persons, however, is that they serve a symbolic function in justifying the imprisonment of the other 99%. The insanity defense, as Stone (1975) has noted, is "the exception that proves the rule. " By exculpating a relatively few people from being criminally responsible for their behavior, the law inculpates all other law violators as liable for social sanction.