Estrus synchronization

Menstrual synchrony, also called the McClintock effect is an alleged process whereby women who begin living together in close proximity experience their menstrual cycle onsets (the onset of menstruation or menses) becoming more synchronized together in time than when previously living apart. "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."

Martha McClintock's 1971 paper, published in Nature, says that menstrual cycle synchronization happens when the menstrual cycle onsets of two or more women become closer together in time than they were several months earlier..

After the initial studies, several papers were published reporting methodological flaws in studies reporting menstrual synchrony including McClintock's study. In addition, other studies were published that failed to find synchrony. The proposed mechanisms have also received scientific criticism. A 2013 review concluded that menstrual synchrony likely does not exist.

published the first study on menstrual synchrony among women living together in dormitories at Wellesley College, a women's liberal arts college.

McClintock hypothesized that pheromones could cause menstrual cycle synchronization. However, other mechanisms have been proposed, most prominently synchronization with lunar phases.

No scientific evidence supports the lunar hypothesis and doubt has been cast on pheromone mechanisms.

After the initial studies reporting menstrual synchrony began to appear in the scientific literature, other researchers began reporting the failure to find menstrual synchrony in women.

These studies were followed by critiques of the methods used in early studies, which argued that biases in the methods used produced menstrual synchrony as an artifact.

More recent studies, which took into account some of these methodological criticisms, failed to find menstrual synchrony.

The term synchrony has been argued to be misleading because no study has ever found that menstrual cycles become strictly concordant, nevertheless menstrual synchrony is used to refer the phenomenon of menstrual cycle onsets becoming closer to each other over time.

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