MENTAL DISORDER
Any illness with a psychological origin manifested either in symptoms of emotional distress or in
abnormal behaviour.
Most mental disorders can be broadly classified as either psychoses or neuroses (see neurosis;
psychosis). Psychoses (e.g., schizophrenia and bipolar disorder) are major mental illnesses
characterized by severe symptoms such as delusions, hallucinations, and an inability to evaluate
reality in an objective manner. Neuroses are less severe and more treatable illnesses, including
depression, anxiety, and paranoia as well as obsessive-compulsive disorders and post-traumatic
stress disorders. Some mental disorders, such as Alzheimer disease, are clearly caused by
organic disease of the brain, but the causes of most others are either unknown or not yet verified.
Schizophrenia appears to be partly caused by inherited genetic factors. Some mood disorders,
such as mania and depression, may be caused by imbalances of certain neurotransmitters in the
brain; they are treatable by drugs that act to correct these imbalances (see psychopharmacology).
Neuroses often appear to be caused by psychological factors such as emotional deprivation,
frustration, or abuse during childhood, and they may be treated through psychotherapy. Certain
neuroses, particularly the anxiety disorders known as phobias, may represent maladaptive
responses built up into the human equivalent of conditioned reflexes.