mental disorder, any illness with significant psychological or behavioral manifestations that is associated with either a painful or distressing symptom or an impairment in one or more important areas of functioning.

(Read Sigmund Freud’s 1926 Britannica essay on psychoanalysis.)

Mental disorders, in particular their consequences and their treatment, are of more concern and receive more attention now than in the past. Mental disorders have become a more prominent subject of attention for several reasons. They have always been common, but, with the eradication or successful treatment of many of the serious physical illnesses that formerly afflicted humans, mental illness has become a more noticeable cause of suffering and accounts for a higher proportion of those disabled by disease. Moreover, the public has come to expect the medical and mental health professions to help people obtain an improved quality of life in terms of mental as well as physical functioning. And indeed there has been a proliferation of both pharmacological and psychotherapeutic treatments. The transfer of many psychiatric patients, some still showing conspicuous symptoms, from mental hospitals into the community has also increased the public’s awareness of the importance and prevalence of mental illness.

There is no simple definition of mental disorder that is universally satisfactory. This is partly because mental states or behaviors that are viewed as abnormal in one culture may be regarded as normal or acceptable in another, and in any case it is difficult to draw a line clearly demarcating healthy from unhealthy mental functioning.

A narrow definition of mental illness would insist upon the presence of organic disease of the brain, either structural or biochemical. An overly broad definition would define mental illness as simply being the lack or absence of mental health—that is to say, a condition of mental well-being, balance, and resilience in which the individual can successfully work and function and in which the individual can both withstand and learn to cope with the conflicts and stresses encountered in life. A more generally useful definition ascribes mental disorder to psychological, social, biochemical, or genetic dysfunctions or disturbances in the individual.

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A mental illness can have an effect on every aspect of a person’s life, including thinking, feeling, mood, and outlook and such areas of external activity as family and marital life, sexual activity, work, recreation, and management of material affairs. Most mental disorders negatively affect how individuals feel about themselves and impair their capacity for participating in mutually rewarding relationships.

Psychopathology is the systematic study of the significant causes, processes, and symptomatic manifestations of mental disorders. The meticulous study, observation, and inquiry that characterize the discipline of psychopathology are, in turn, the basis for the practice of psychiatry (i.e., the science and practice of diagnosing and treating mental disorders as well as dealing with their prevention). Psychiatry, psychology, and related disciplines such as clinical psychology and counseling embrace a wide spectrum of techniques and approaches for treating mental illnesses. These include the use of psychoactive drugs to correct biochemical imbalances in the brain or otherwise to relieve depression, anxiety, and other painful emotional states.

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Another important group of treatments is the psychotherapies, which seek to treat mental disorders by psychological means and which involve verbal communication between the patient and a trained person in the context of a therapeutic interpersonal relationship between them. Different modes of psychotherapy focus variously on emotional experience, cognitive processing, and overt behavior.

This article discusses the types, causes, and treatment of mental disorders. Neurological diseases (see neurology) with behavioral manifestations are treated in nervous system disease. Alcohol and other substance use disorders are discussed in alcoholism and drug use. Disorders of sexual functioning and behavior are treated in sexual behavior, human. Tests used to evaluate mental health and functioning are discussed in psychological testing. The various theories of personality structure and dynamics are treated in personality, while human emotion and motivation are discussed in emotion and motivation. See also personality disorder; psychopharmacology; psychotherapy.