Culture and menstruation

The word "menstruation" is etymologically related to "moon". The terms "menstruation" and "menses" are derived from the Latin mensis (month), which in turn relates to the Greek mene (moon) and to the roots of the English words month and moon.

To many, such cultural associations appear persuasive in view of the fact that in humans, the menstrual cycle quite closely approximates the moon’s 29.5-day synodic cycle, unlike in chimpanzees (~36 days) or bonobos (~40 days). Statistical information from hunter gatherers is lacking, but where large-scale western studies focus on women’s peak reproductive years – removing outlier values – the cycle length gravitates around 29.1-29.5 days, while the figure for women in their thirties shortens toward 28 days. In no extant human population has statistically significant lunar phase-locking been demonstrated. Turning to the evolutionary past, however, a possible adaptive basis for the biological capacity would be reproductive levelling: among primates, synchronising to any natural clock makes it difficult for an alpha male to monopolise fertile sex with multiple females. This would be consistent with the striking gender egalitarianism of extant non-storage hunter-gatherer societies. A further deep-time evolutionary pressure may have been lions’ habit of eating people on moonless nights. When early Pleistocene hominids in Africa were attempting to survive by robbing big cats of their kills, according to some evolutionary scientists, it may have been adaptive to restrict overnight journeys – including sexual liaisons – to times when there was a moon in the sky.

Menstrual synchrony, in particular by association with the moon, is a belief found in mythology throughout the world. The idea that menstruation is – or ideally ought to be – in harmony with wider cosmic rhythms is one of the most tenacious ideas central to the myths and rituals of traditional communities across the world. One of the most thoroughgoing analyses of primitive mythology ever undertaken was that of the French anthropologist Claude Lévi-Strauss, who concluded that, taken together, the indigenous myths of North and South America expressed men's worry that, unless women's periods were carefully monitored and synchronised, the universe might descend into chaos.

The !Kung (or Ju|'hoansi) hunter-gatherers of the Kalahari “believe....that if a woman sees traces of menstrual blood on another woman’s leg or even is told that another woman has started her period, she will begin menstruating as well”. Among the Yurok Indians of northwestern California, according to one ethnographic study, "all of a household’s fertile women who were not pregnant menstruated at the same time…"

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