Culture and menstruation the ideal of synchronised tidal, lunar, menstrual and seasonal periodicities whose overall harmony (it is believed) confers spiritual power and fertility. 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
Menstrual cycle mating partners during the menstrual cycle; however, those seeking a short-term relationship were more likely to choose a partner with more masculine features than usual. This was found to be the case especially during the woman's high conception risk stage and when salivary testosterone was high. However, when women are in the luteal (non-fertile) phase, they tend to prefer men (and females) with more feminine faces. A preference is also shown for self-resembling faces and apparent health in faces during the luteal phase of the cycle. Apparent health preferences were found to be strongest when progesterone levels were high. Additionally,