As humans, we are constantly trying to find ways to make sense of ourselves, others, and the world around us. One way we do this is by assigning names to things, be it physical objects, concepts, or experiences. We designate things to to categories, providing a semblance of order. We look for frameworks to give us a way of understanding, or perhaps a common language.

Psychological distress is no exception.

Classifying mental illness: a brief history

From the Latin term “insania” for insanity to the use of “mania” and "melancholia" in Greek to denote a chaotic frenzy and depression, there has been a rich vocabulary used to discuss mental illness since ancient times. But the first attempt to classify such afflictions was by the Greek physician Hippocrates in 400 BC, who believed that mental illness stemmed from imbalances of a person's black bile, yellow bile, phlegm, and blood. Different imbalances resulted in particular symptoms which could be split into the following categories: Mania, Melancholy, Phrenitis (brain inflamation), Insanity, Disobedience, Paranoia, Panic, Epilepsy, and Hysteria.

Inspired by botanical taxonomy (the classification of plants), a French physician called François Boissier de Sauvages de Lacroix published a system of classifying illness in 1763. This classification included mental illnesses, subdivided into four categories: 1) Hallucinations, 2) Morositates, 3) Deliria, and 4) Folies Anomales. Within these categories were some familiar symptoms including induced vomiting, mania, amnesia, hypersexuality, panic, and insomnia. Other symptoms, like "the uncontrollable impulse to dance" and "non-aggressive delirium with accompanying sadness caused by the devil" are a world away from the Western psychiatric manuals we know today.

The 19th century saw more attempts to classify mental illness. In Germany, Karl Kahlbaum published his 'Classification of Psychiatric Diseases and Mental Disturbances' (1863), positing a system which classified mental illnesses by their symptoms. Rejecting the tradition of labelling a symptom as a particular illness, he conceptualised psychiatric diagnoses as clusters of symptoms: mania as a symptom of a disorder instead of a disorder in itself. Kahlbaum employed many terms that we still use today including Dysthymia, Cyclothymia, Catatonia, Paranoia, and Hebephrenia. Inheriting Kahlbaum's ideas, Emil Kraeplin, in the late 19th and early 20th century, proposed a system in which a disorder was defined not only by the symptoms that constitute it, but also by the patterns and course in which it presents. Famously, he differentiated between Psychotic Disorders and Affective Disorders, providing the foundations for what we now refer to as Schizophrenia and Bipolar Disorder.

Today, the two most widely established systems of psychiatric classification are the Diagnostic and Statistical Manuel of Mental Disorders (DSM) and the International Classification for Diseases (ICD). Despite each being as widely used as the other, the ICD and the DSM conceptualise and classify mental disorders in different ways.

What is the Diagnostic and Statistical Manual of Mental Disorders (DSM)?

The DSM is published by the American Psychiatric Association, America's main professional organisation of psychiatrists. It is the world's largest psychiatric organisation, with upwards of 38,500 members in over 100 countries. Given that the DSM only includes Mental Disorders, it is used primarily by Psychiatrists but also by other mental health professionals. The beginnings of the DSM arose before the APA went by its current name, then called the Committee on Statistics of the American Medico-Psychological Association. In 1917, it published the 'Statistical Manual for the Use of Institutions for the Insane' which outlines the symptoms of 21 disorders. All the disorders were, bar two, psychotic in nature.

Reformulated as the DSM, the first edition was released in 1952 and contained 128 categories. It differentiated between organic brain syndromes and functional (physically undetectable) disorders. Functional disorders were further divided into Psychotic Disorders, Neurotic Disorders (distress without psychosis), and Personality Disorders. Descriptions were short, leaving it up to the diagnosing clinician's discretion to interpret meaning, focusing on the cause of disorders rather than their symptoms. In this way, the manual honoured the psychodynamic tradition. Whilst the first DSM- seemed to be aimed at diagnosing patients in psychiatric hospitals, the DSM-2 (1968) was thought to have more relevance to outpatients. The sections on Depressive Disorders, Anxiety Disorders, and Personality Disorders were expanded in this edition, and disorders specific to children and adolescents - as well as a miscellaneous category - were introduced. The seventh printing of DSM-2 saw homosexuality depathologised, reflecting a departure from mental illness as being a deviation from accepted social values.

The DSM-3 marked a paradigm shift: a move away from vague descriptions mostly concerned with a disorder's origin, instead embracing the clinical specificity of diagnostic criteria. With the DSM-3 (1980) came the introduction of the multi-axial system. The multi-axial system meant that a patient would be diagnosed through information concerning fives separate axes: (I) clinical disorders such as Psychotic Disorders, Mood Disorders, or Anxiety Disorders, (II) Personality Disorders and Developmental Delays, (III) physiological medical disorders that have relevance to the patient's psychiatric presentation in terms of affecting functioning or mood, or impacting medication choices, (IV) psychosocial stressors in the patient’s environment, (V) assessment of the patient’s general ability to function. The practice of adding "not otherwise specified" onto the category name of disorder was first seen in this manual, meant as a way to indicate when a patient met many of the diagnostic criteria without this being sufficient to obtain the particular diagnosis. DSM-3 was revised seven years after its initial publication and the DSM-3-R was released, changing diagnostic criteria to reflect the newest research. The reformulation also did away with exclusion criteria, leaving behind a hierarchical structure and simplifying the task of diagnosis.

Published in 1994, the DSM-4 built upon the clinical research generated for its predecessor, with most diagnoses being grounded in at least some research. Using a symptom-based method of classification meant the manual grew considerably in length. Field trials for it recruited participants from a variety of ethnic and cultural backgrounds, demonstrating a new-found concern for cross-cultural validity. In the same vein, culture-specific disorders were included. DSM-4 was revised in 2000, with corrections made to factual errors and research updated to reflect that which was most recently published.

The DSM-5, published in 2013, is the most up-to-date manual. Like the DSM-4, it is based upon the work of expert study groups and makes use of large sets of data. Notably, the multi-axial system was abandoned; the axes I, II, and III were amalgamated to form Psychiatric and Medical Diagnoses; psychosocial considerations were incorporated into the descriptions of the disorders; and Axis V, the assessment of general functioning, was dropped due to "conceptual lack of clarity"(APA, 2013). Changes in the DSM-5 include the diagnosis of Autism Spectrum Disorder subsuming what were previously separate disorders and the inclusion of gender dysphoria. DSM-5 also seeks to order categories of disorders in a way that reflects aetiological similarity. For example, Trauma- and Stressor-Related Disorders fall next to Dissociative Disorders, acknowledging that all Dissociative Disorders but one are necessarily traumagenic. Anxiety Disorders precede Obsessive-Compulsive and Related Disorders, reflecting current research which posits their similarity but also meaningful distinction. The DSM-5 is a polythetic system in that lists of symptoms are given and diagnostic labels are assigned to patients based upon whether the specified symptoms - or sometimes number of symptoms - are met.

What is the International Classification for Diseases (ICD)?

The other dominant diagnostic manual is the ICD. Born from the 'International List of Causes of Death', adopted in 1893 by the International Statistical Institute, the ICD is used to compile morbidity and mortality statistics. The 'International List of Causes of Death' was revised five times since initial publication, with the sixth version being rebranded as the ICD in 1948. The ICD is created by the World Health Organisation (WHO), an agency of the United Nations that is concerned with worldwide public health. Translated into 43 languages, the ICD is used in over 100 countries. Whilst the DSM is concerned only with psychiatric disorders, the ICD includes all health disorders so is utilised by a variety of medical professionals.

The ICD-6 was different to its predecessors in that it chronicled not just mortality, but also morbidity. It included a chapter titled Mental, Psychoneurotic, and Personality Disorders which included three sections: Psychoses, Psychoneurotic Disorders, and Disorders of Character, Behavior, and Intelligence. Across these sections were 26 categories, each containing multiple diagnoses. For example, within the Psychoses section was Schizophrenic disorders (dementia praecox), which contained the subcategories Simple type, Hebephrenic type, Catatonic type, Paranoid type, Acute schizophrenic reaction, Latent Schizophrenia, Schizo-Affective Psychosis, and Other/Unspecified. These subcategories contained the diagnoses themselves; the Simple type category contained Schizophrenia and Dementia, in both simple and primary forms. There were no descriptions accompany the diagnoses, just identifying numbers.

Apart from amendments of errors, no changes were made from the ICD-6 to the ICD-7. In 1968, the ICD-8 came into effect. The ICD-8 retained the psychoses section of ICD-7 but grouped Neuroses, Personality Disorders, and other Nonpsychotic mental disorders together, moving what was known as Mental Retardation to its own section. The WHO published a glossary of terms in 1974, designed to elucidate a greater understanding of diagnostic categories given that the ICD-8 still lacked descriptions. "It has become increasingly obvious that many key psychiatric terms are acquiring different meanings in different countries", reads the glossary, highlighting the need for standardisation. The glossary took a descriptive, symptom-based approach, and the ICD-9 (1979) incorporated much of it. The ICD-9-CM was created by the U.S. National Centre for Health Statistics, based on the ICD-9 but with additional morbidity details. CM stands for Clinical Modification, and this adaptation was created specifically for use in the American healthcare system.

Since 1994, the ICD-10 has been in use, and is perhaps the most radical change between versions of the manual yet. Like the previous version, it has a clinically modified version for use in America. The ICD-10 departs from the traditional division between neurosis and psychosis that the ICD-9 recognised. Instead, disorders are arranged in groups by likeness. For example, Cyclothymia is no longer under the category of Personality and Behavioural Disorders, it is now in Mood (Affective) Disorders, reflecting our current understanding of the disorder. In the ICD-10, the WHO attribute the greater detail relating to Behavioural Syndromes and Mental Disorders Associated with Physiological Dysfunction and Hormonal Changes to the increasing use of liaison psychiatry. Mental and Behavioural Disorders relating to substance use are rearranged, now detailing both the substance used and what characterises the coexisting syndrome. Schizophrenia, Schizotypal, and Delusional Disorders have been expanded to include Undifferentiated Schizophrenia, Post-Schizophrenic Depression, and Schizotypal Disorder. A new subcategory of Disorders of Adult Personality and Behaviour is in the ICD-10; it concerns exaggeration or total fabrication of symptoms, either psychological or physical. Developmental Disorders, such as Autism, are classified as Pervasive Developmental Disorders in the ICD-10 whereas the previous manual saw them labelled as Psychoses.

The ICD-11 is set to come into effect in 2022. There are many changes to the latest update, available online, with additional diagnostic codes offering a greater level of specificity. Being transgender will no longer be considered a mental disorder: a change that reflects a contemporary understanding of gender identity. Gaming Disorder will now be a diagnosable condition, with the NHS already commencing specialised treatment for it, as will Attention-Deficit Hyperactivity Disorder (ADHD). What used to be known as "Enduring Personality Change After Catastrophic Event" is going to be reformulated as Complex Post-Traumatic Stress Disorder (CPTSD), and a Prolonged Grief Disorder will be added. Perhaps the biggest shakeup is with regards to Personality Disorders; someone will be diagnosed with a primary personality dysfunction, with a signifier of severity, along with any additional traits. The ICD-11 does not provide operational criteria for disorders, but rather descriptions.

These changes have not all been well-received by both clinicians and psychiatric patients, but that's a discussion for another article.


This is the first article in a series about the DSM vs. the ICD: the tensions, politics, & practical applications of understanding mental distress though two 'competing' diagnostic manuals.

Next article: how do the two systems of diagnosis interact? Do "equivalent" diagnoses exist?