While in the majority of women the menstrual cycle is regular for the individual, and corresponds to the lunar month of 28 days, it must be added that in a considerable minority it is rather longer, or, more usually, shorter than this, and in many individuals is not constant. Osterloh found a regular type of menstruation in 68 per cent, healthy women, four weeks being the most usual length of the cycle; in 21 per cent, the cycle was always irregular. See Näcke, "Die Menstruation und ihr Einfluss bei chronischen Psychosen," Archiv für Psychiatrie, 1896, Bd, 28, Heft 1.
Among the Duala and allied negro peoples of Bantu stock dances of markedly erotic character take place at full moon. Gason describes the dances and sexual festivals of the South Australian blacks, generally followed by promiscuous intercourse, as taking place at full moon. (Journal of the Anthropological Institute, November, 1894, p. 174.) In all parts of the world, indeed, including Christendom, festivals are frequently regulated by the phases of the moon.
It has often been held that the course of insanity is influenced by the moon. Of comparatively recent years, this thesis has been maintained by Koster (Ueber die Gesetze des periodischen Irreseins und verwandter Nervenzustände, Bonn, 1882), who argues in detail that periodic insanity tends to fall into periods of seven days or multiples of seven.