Abstract
Defining disease and disorder remains a key conceptual question in philosophy of medicine and psychiatry, and is currently a very practical matter for psychiatric nosology, given the new Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, and the upcoming International Classification of Diseases, 11th Revision. There have been advances in the cognitive-affective science of human categorization, and it is timely to consider implications for our understanding of the category of psychiatric disorder. The category of mental disorder has graded boundaries, and conditions within this category can be conceptualized using MEDICAL or MORAL metaphors. One key set of constructs used in MEDICAL metaphors relates to the notion of dysfunction, and it may, in turn, be useful to conceptualize such dysfunction in evolutionary terms. For typical disorders, it is relatively easy to agree that dysfunction is present. However, for atypical disorders, there may be considerable debate about the presence and extent of dysfunction. Rational arguments can be brought to bear to help decide whether particular entities should be included in our nosologies, and, if so, what their boundaries should be. However, it is appropriate that there should be ongoing debate on diagnostic validity, clinical utility, and other relevant facts and values, for cases that are difficult to decide. The perspective here can be illustrated using many nosological debates within the anxiety disorders and the obsessive-compulsive and related disorders, including the question of delineating normal from abnormal anxiety, of deciding whether anxiety is psychiatric or medical, and the debate about the optimal meta-structure for anxiety disorders.