Bisexuality and biorhythm: Wilhelm Fliess’s theory of the rhythm of living substance

Abstract Biorhythm is a current pseudoscience used to predict human performance on a particular day, based on the number of days since birth. Hawked as an aid to online gambling, it was a popular-science fad in 1970s America, based on German popularizations of a vitalistic biological theory articulated at the turn of the century by Wilhelm Fliess. A friend and confidant of Sigmund Freud, Fliess hypothesized a duality of masculine and feminine substances in every human, which were expressed at rhythmic intervals, producing two series of critical days on which major biological events were more likely than at other times. As scientific research on biological rhythmicity during the inter-war period pointed to the existence of internal sources of timing in plants and animals and the rhythmicity of life entered popular-cultural discussion, Fliess’s theory was transformed into the basis for biorhythm, through a reinterpretation of the rhythmic nature of his intervals.