Classification and Diagnostic Systems

The defining criteria for intellectual disabilities (ID) and psychopathology have undergone continuous development since the first attempts to define these concepts. Diagnostic systems include those from the American Psychiatric Association, the American Association on Intellectual and Developmental Disabilities, and the World Health Organization’s International Classification of Diseases 11th edition. Over time, these diagnostic categories have evolved from relatively simple and rigid to complex, multidimensional categories. The chapter sections “Overview” and “Shifting Definitions of Intellectual Disabilities” explore historical and contemporary definitions of ID and psychopathology. While the consideration of possible interactions between ID and psychopathology has an established history, researchers did not dedicate major efforts to understanding dual diagnosis until the 1980s. Current concepts and measurements of dual diagnosis, as well as diagnostic dilemmas, are explored in the “Definitions of Psychopathology” section of this chapter.