Menstrual synchrony by chance when there is menstrual cycle variability. Yang and Schank argued that when there is cycle variability (i.e., either women have irregular cycles, have cycles of different frequencies, or both), most women will have the opportunity to experience synchrony even though it is a result of cycle variability and not a result of a mechanism such as the exchange of pheromones. For example, consider two women A and B. Suppose A has menstrual cycles that are 28 days long and B has cycles that are 30 days long. Suppose further that when A and B become close friends, B
Menstrual synchrony synchrony throughout an inter-birth interval, Power et al. argued that once we take account of birth seasonality – enhancing the effects of menstrual synchrony by clumping fertile cycles within a relatively brief time-window – it emerges that reproductive synchrony can be effective as a female strategy to undermine primate-style sexual monopolization by dominant males. The controversy remains unresolved. Adopting a compromise position, one school of Darwinian thought argues that the human female may once have had adaptive reasons for evolving a 29.5-day menstrual cycle – implying some theoretical potential for synchrony to a lunar clock – but did so in