Kraepelin's classification of mental illness

By the late nineteenth century European psychiatrists had described several disorders of unknown cause that struck the young and often ended in lasting decline. In France Morel called such cases démence précoce, and in Scotland Clouston wrote of adolescent insanity. In Germany Kahlbaum described catatonia, and his pupil Hecker described hebephrenia.

Emil Kraepelin (1856 to 1926), professor of psychiatry and director of the psychiatric clinic at the University of Heidelberg, followed large numbers of patients over time and grouped them by how their illness ran and ended. His textbook, first published in 1883, changed from edition to edition. In the sixth edition, published in Leipzig in 1899, he divided the major psychoses into two groups. Dementia praecox had a chronic course with steady cognitive decline. Manic-depressive insanity ran in episodes, with relatively little lasting deterioration.

He gathered catatonic, hebephrenic and paranoid forms into dementia praecox as its subtypes, and he built manic-depressive insanity from the long-described pictures of mania and depression. He never issued a list of diagnostic criteria and avoided naming any single symptom as decisive, expecting brain pathology, physiology and chemistry eventually to confirm his categories.

The Swiss psychiatrist Eugen Bleuler renamed dementia praecox schizophrenia in a 1911 monograph and widened it to include patients who did not deteriorate, calling it a group of diseases. Kraepelin himself wrote in 1920 that "our formulation of the problem may be incorrect" and considered replacing the two categories with overlapping registers of symptoms.

The two-way split outlasted his doubts. Diagnostic systems still separate schizophrenia from bipolar disorder, although a 2019 review of the dichotomy found genetic, treatment and outcome evidence both for the division and for a zone of overlap between the two illnesses.