Chapter 1 Issues in Diagnosis : Categorical vs . Dimensional

Psychiatric diagnosis is fundamental to the understanding of mental illness. Without it, the study, assessment, and treatment of psychopathology would be in disarray. In this chapter, we examine: (1) the raison d’etre underlying psychiatric diagnosis; (2) widespread misconceptions regarding psychiatric diagnosis; (3) the present system of psychiatric diagnosis and its strengths and weaknesses; and (4) and fruitful directions for improving this system. There are a myriad of forms of abnormality housed under the exceedingly broad umbrella of mental disorders. Indeed, the current psychiatric classification system contains well over 350 diagnoses (American Psychiatric Association [APA], (2000)). The enormous heterogeneity of psychopathology makes a formal system of organization imperative. Just as in the biological sciences where Linnaeus’ hierarchical taxonomy categorizes fauna and flora and in chemistry where Mendeleev’s periodic table orders the elements, a psychiatric classification system helps to organize the bewildering subforms of abnormality. Such a system, if effective, permits us to parse the variegated universe of psychological disorders into more homogeneous, and ideally more clinically meaningful, subtypes. From the practitioner’s initial inchoate impression that a patient’s behavior is aberrant to later and better elaborated case conceptualization, diagnosis plays an integral role in the clinical process. Indeed, the essential reason for initiating assessment and treatment is often the observer’s sense that ‘‘something is just not quite right’’ about that person. Meehl (1973) commented that the mental health professional’s core task is to answer the question, ‘‘what does this person have, or what befell him, that makes him different