The monthly rhythm of incidence and age at menarche: thirty fi ve years of research. The circa-vacation-study expectancy rhythm of incidence and age at menarche

Abstract

The hypothesis that the vacation-study-expectancy scholar regime produces most of the monthly rhythm of the age at menarche (AaM) was tested. Studies on monthly menarche incidence (MI) refuted climatic factors as a main factor in this rhythm, and indicated that the main factor of this rhythm is the succession of expectancies of study (Stu-months) or vacation (Vac-months) months within a year. Thus the hypothesis of seasonal circa-annual rhythm should be modifi ed to the circa-[vacation (fi esta)]-[study (non-fi esta)]-expectancies rhythm for the MI and age at menarche annual rhythms. In several countries Vac-months had higher MI than Stu-months. The high MI of Vac-months was followed by a large decrease when girls started their studies and a MI increase occurred as vacations approached. The hypothesis proposes that at the end of vacations and at the beginning of the study period the AaM should be lowest, and then the mean of AaM should increase because of the menarche delay of girls whose menarche was arrested by the initiation of school work. This pattern was found in four independent samples, from Chile, Colombia, USA and Brazil. The probability that this result be due to random fl uctuation of means is extraordinarily low (P<10-8). I conclude that the infl uence of the expectancy of vacation and study periods on the monthly rhythm of the age at menarche is a real process that accounts for most of this rhythm.