Menstrual synchrony
![Group synchrony scores over time with 99% confidence intervals from McClintock's study (an approximation of Fig 1).[3] In October, the mean onset difference was about 6.5 days and by the end of the study the mean onset difference decreased by almost 2 days.](/uploads/202501/26/McClintock_1971_Fig_1_Nature_group_synchrony2230.jpg)
![An illustration of menstrual cycle convergence and divergence as described in Yang and Schank.[12] For illustration, A and B have menses duration of four days each. A has 28 day cycles and B has 30 day cycles. The first onset of B that appears is 14 days earlier than A's. Red vertical lines indicate menses overlap or meet. In this example, cycles gradually converge and A and B may](/uploads/202501/26/Yang_and_Schank_2006_converging_diverging_cycles22231.jpg)
Menstrual synchrony, also called the McClintock effect, is the alleged process whereby women who begin living together in close proximity experience their menstrual cycle onsets (the onset of menstruation or menses) becoming closer together in time than previously. "For example, the distribution of onsets of seven female lifeguards was scattered at the beginning of the summer, but after 3 months spent together, the onset of all seven cycles fell within a 4-day period."