Psychiatric nosology arose in central Europe towards the end of the 19th century, in particular with Kraepelin’s foundational distinction between dementia praecox (schizophrenia) and manic depressive insanity.1 The distinction between bipolar illness and unipolar (major) depression was first proposed in the late 1950s and became increasingly widely accepted. The major syndromes—especially schizophrenia, bipolar disorder, and major depression—were differentiated on the basis of their symptom patterns and course of illness. At the same time, clinical features such as psychosis, mood dysregulation, and cognitive impairments were known to transcend diagnostic categories. Doubt remains about the boundaries between the syndromes and the degree to which they signify entirely distinct entities, disorders that have overlapping foundations, or different variants of one underlying disease. Such debates have intensified with syndromes described subsequently, including autism spectrum disorders and attention deficit-hyperactivity disorder.
