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 (a) the raison d'ĂȘtre underlying psychiatric diagnosis, (b) widespread misconceptions regarding psychiatric classification and diagnosis, (c) the present system of psychiatric classification and its strengths and weaknesses, and (d) fruitful directions for improving this system.
A myriad of forms of abnormality are housed under the exceedingly broad umbrella of mental disorders. Indeed, the current psychiatric classification system, as described in the Diagnostic and Statistical Manual of Mental Disorders (DSMâ5), contains well over 300 diagnoses (American Psychiatric Association [APA], 2013). In addition, Chapter V of the International Classification of Diseases, 10th edition (ICDâ10), contains approximately 100 diagnoses housed within 26 categories (World Health Organization, 1992). The enormous heterogeneity of psychopathology makes a formal system of organization imperative. Just as in the biological sciences, where Linnaeus's 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, categories.
From the practitioner's initial inchoate impression that a patient's behavior is aberrant to his or her later and betterâelaborated 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 âsomething is just not quite rightâ about the person. Meehl (1973) commented that the mental health professional's core task is to answer the question: âWhat does this person have, or what befell him, that makes him different from those who have not developed clinical psychopathology?â (p. 248). Therein lies the basis for psychiatric diagnosis.
General Terminological Issues
Before proceeding, a bit of terminology is in order. It is crucial at the outset to distinguish two frequently confused terms: classification and diagnosis. A system of classification is an overarching taxonomy of mental illness, whereas diagnosis is the act of placing an individual, based on a constellation of signs (observable indicators, like crying in a depressed patient) or symptoms (subjective indicators, like feelings of guilt in a depressed patient), or both, into a category within that taxonomy. Classification is a prerequisite for diagnosis.
Another key set of terminological issues concerns the distinctions among syndrome, disorder, and disease. As Kazdin (1983) observed, we can differentiate among these three concepts based on our levels of understanding of their pathologyâthe underlying pathophysiology that may accompany the conditionâand etiology, that is, causation (Gough, 1971; Lilienfeld, Waldman, & Israel, 1994).
At the lowest rung of the hierarchy of understanding there are syndromes, which are typically constellations of signs and symptoms that coâoccur across individuals (syndrome means ârunning togetherâ in Greek). In classical syndromes, neither pathology nor etiology is well understood, nor is the syndrome's causal relation to other conditions established. Antisocial personality disorder is a relatively clear example of a syndrome because its signs (e.g., the use of an alias) and symptoms (e.g., lack of remorse) tend to covary across individuals. Nevertheless, its pathology and etiology are largely unknown, and its causal relation to other conditions is poorly understood (Lykken, 1995). In contrast, some authors (e.g., Lilienfeld, 2013; but see Lynam & Miller, 2012) argue that psychopathic personality (psychopathy) may not be a classical syndrome. These researchers contend that psychopathy is instead a configuration of several largely independent constructs, such as boldness, coldness, and disinhibition, that come together in an interpersonally malignant fashion (Patrick, Fowles, & Krueger, 2009; see also Vitale & Newman, Chapter 15, in this book).
In other cases, syndromes may also constitute groupings of signs and symptoms that exhibit minimal covariation across individuals but that point to an underlying etiology (Lilienfeld et al., 1994). For example, Gerstmann's syndrome in neurology (Benton, 1992) is marked by four major features: agraphia (inability to write), acalculia (inability to perform mental computation), finger agnosia (inability to differentiate among fingers on the hand), and leftâright disorientation. Although these indicators are negligibly correlated across individuals in the general population, they coâoccur dependably following certain instances of parietal lobe damage.
At the second rung of the hierarchy of understanding there are disorders, which are syndromes that cannot be readily explained by other conditions. For example, in the present diagnostic system, obsessiveâcompulsive disorder (OCD) can be diagnosed only if its symptoms (e.g., recurrent fears of contamination) and signs (e.g., recurrent hand washing) cannot be accounted for by a specific phobia (e.g., irrational fear of dirt). Once we rule out other potential causes of OCD symptoms, such as specific phobia, anorexia nervosa, and trichotillomania (compulsive hair pulling), we can be reasonably certain that an individual exhibiting marked obsessions or compulsions, or both, suffers from a wellâdefined disorder (APA, 2000, p. 463).
At the third and highest rung of the hierarchy of understanding there are diseases, which are disorders in which pathology and etiology are reasonably well understood (Kazdin, 1983; McHugh & Slavney, 1998). Sickleâcell anemia is a prototypical disease because its pathology (crescentâshaped erythrocytes containing hemoglobin S) and etiology (two autosomal recessive alleles) have been conclusively identified (Sutton, 1980). For other conditions that approach the status of bona fide diseases, such as Alzheimer's disease, the primary pathology (senile plaques, neurofibrillary tangles, and granulovacuolar degeneration) has been identified, while their etiology is evolving but incomplete (Selkoe, 1992).
With the possible exception of Alzheimer's disease and a handful of other organic conditions, the diagnoses in our present system of psychiatric classifications are almost exclusively syndromes or, in rare cases, disorders (Kendell & Jablensky, 2003). This fact is a sobering reminder that the pathology in most cases of psychopathology is largely unknown, and their etiology is poorly understood (Kendler, 2005; Kendler, Zachar, & Craver, 2011). Therefore, although we genuflect to hallowed tradition in this chapter by referring to the major entities within the current psychiatric classification system as mental âdisorders,â readers should bear in mind that few are disorders in the strict sense of the term.