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, hallucination s, 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 disorder s and post-traumatic stress disorder s. 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 neurotransmitter s 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 phobia s, may represent maladaptive responses built up into the human equivalent of conditioned reflexes.