A group of affective psychotic reactions characterized by a predominant mood of elation or depression, accompanied by related disturbances of thought and activity. The behavioral changes that occur range from relatively mild to extremely severe and uncontrolled reactions, but in most cases they appear to be exaggerations of normal tendencies rather than bizarre distortions.Manic-depressive reactions were observed by the Egyptians, Hebrews, and Greeks. Hippocrates’ descriptions of “melancholia” and “mania” were remarkably similar to our own. As early as the sixth century, Trallianus recognized that the two reactions may alternate in the same person. Th6ophile Bonet used the term “folie maniaco- milancolique in 1684, and Jean Pierre Falret called the disorder “folie circulate" in 1854. Emil Kraepelin introduced the term “manic-depressive psychosis” in 1899, and gave a clear clinical picture of both elation and depression. Recent studies have shown that recurrent manic or depressive episodes are more prevalent than an alternation between the two, and sometimes the two states are found in combined form.The clinical pictures of the various types will be given separately. In general, the manic patient is elated, boisterous, uninhibited, distractible, and hyperactive. He constantly proposes wild schemes, and may have delusions of grandeur. The depressed patient, on the other hand, is despondent, lonely, listless, slow-thinking, hypochondriacal, and often accuses himself of unpardonable sins. About 75 per cent of depressed cases entertain suicidal thoughts, and about 10 to 15 per cent make the attempt. Manic patients sometimes become hostile and assaultive if they are interfered with, but rarely kill anyone. Both episodes tend to be shorter than other psychotic reactions. On the average, the untreated depressive phase runs its course in nine months, and the manic phase in about three months— but the range is from a few days to a year. The episodes are briefest among adolescents and young adults. Therapy will shorten their duration in practically all cases, but there is a distinct tendency for them to recur; only 25 per cent have one attack. After an episode, the patient returns to normality and shows no evidence of mental deterioration.Due largely to successful treatment in clinics, the rate of first admissions to public mental hospitals has dropped from 6 to 2 per cent within the past fifteen years. The rate for private clinics and hospitals, particularly for depressive cases, appears to be much higher. Most cases occur between the ages of twenty- five and sixty-five, and the median age of first admissions is forty-four. Manic episodes generally start earlier in life than depressive episodes, but are less likely to become chronic. In either case, the earlier the onset, the poorer the prognosis. Female patients outnumber male by about four to three.Reports on the socioeconomic distribution of the disease are conflicting; some studies in this country indicate that it is most common on the lower, others on the higher occupational levels. The incidence is considerably higher in urban than in rural areas. Forty-five per cent of the hospital cases are depressed, 30 per cent manic, and 25 per cent mixed and circular—that is, alternating. For details on these types, as well as on etiology, therapy, and prognosis, see entries that follow."