The Architecture of Traumatic Time
POST-TRAUMATIC STRESS DISORDER is part of a monothetic system of classification. Within this system, each classification (disorder) is identified with a list of criterial features that are individually necessary and collectively sufficient for including or excluding a case from the classification. A case that overlaps the boundaries between classifications is handled in two ways. Either the patient is given concurrent diagnoses—the case is diagnosed as belonging to both categories X and Y—or the patient is said to have a “mixed disorder,” which includes features of two different disorders, such as schizo-affective disorder.
DSM-III included one entry, schizotypal personality disorder, that did not conform to the monothetic rule. To qualify as a schizotypal personality, a patient needed to exhibit any four features from a list of eight. This is a polythetic classification, meaning that it is possible for two correctly diagnosed cases to have no features in common. In DSM-III-R, the number of polythetic classifications expanded, and it now includes the personality disorders, behavior disorders, and chemical substance use disorders. While DSM-III did not call attention to its departure from the monothetic rule, DSM-III-R does (Amer. Psychia. Assoc. 1987:xxiv; Livesley 1985:355).
When, in its introduction, DSM-III-R discusses polythetic classifications, it is referring to an explicitly polythetic system (Widiger and Frances 1988:615). In order for a case to qualify as a member of a polythetic diagnostic category, it needs only to cross an indicated threshold value. Membership in the classification is based on overlapping features (family resemblances), as in the following example, where cases get the same diagnosis (“X”) if they possess three out of six attributes (A to F):