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Menstrual Synchrony

In XX century scientists observed very specific phenomenon wherein the menstrual cycles of females (girls and/or women) living together or living in close quarters were synchronized – actually women will have menstrual bleeding almost at the same time and menstrual cycles were working synchronized. This phenomenon was called “menstrual synchrony” – it has been found in room-mates, sisters, close friends, lesbian couples and most strongly between mothers and daughters. Menstrual synchrony was also observed in women prisons, convents, bordellos and/or dormitories. Same phenomenon has also been noted in mice, hamsters and rats.

It was suggested that ovulation (process of releasing egg from ovary) could be somehow “socially regulated” and synchronized ovulation could lead to so called “menstrual synchrony”. Same phenomenon has several names including “menstrual synchronicity”, “menstrual synchrony”, “synchronized periods”, “dormitory effect “, “McClintock effect”.

McClintock effect

The phenomenon about women’s synchronized periods was called as the “McClintock effect” for the author of the first article published called “Menstrual Synchrony and Suppression”, by Martha McClintock. This article was published in Nature in 1971 and contains the discussion about synchronized periods in women living together or in very close distances.

Actually earlier Ms. McClintock participated at scientific conference where the phenomenon of synchronized ovulation in mice was discussed and Ms. McClintock commented that the same thing happened in human females as well, as evidenced in dormitory life. While at first wary of the assertion, the scientists challenged her, as an undergraduate, to address the issue scientifically. McClintock did just that when she pursued the topic for her senior thesis at Wellesley and then published it when she was working on her graduate degree at Harvard.

This was the first paper to suggest any operation of pheromones in humans.

At that time it did not definitively prove that pheromones were acting causally but Ms. McClintock’s paper did offer tantalizing evidence. Further research by McClintock and others (Russell et al., 1980; Stern and McClintock, 1988; etc) did prove that it was pheromones causing the synchrony. Other studies have also found influences of male pheromones on women’s menstrual cycles as well as evidence of female pheromones effecting men’s hair growth.

Menstrual Synchrony

Menstrual Synchrony

Synchronized periods known as the McClintock effect, also known as menstrual synchrony or the dormitory effect, is a theory that proposes that the menstrual cycles of women who live together (such as in prisons, convents, bordellos, or dormitories) tend to become synchronized over time.

It is thought to be analogous to the Whitten effect, which is the synchronization of the estrous cycle and has been noted in small animals such as mice and guinea pigs. In contrast to the Whitten effect, which is driven by male pheromones, the McClintock effect is postulated to have only female pheromonal involvement.

Mechanism of Menstrual Synchrony

It was discovered that pheromones (hormone-like chemicals) are released by specific skin glands concentrated under the arm – these are airborne chemicals which are not detected as odor by the nose but they are sensed in the nose by vomeronasal organ (VNO, so called Jacobson’s organ). When woman is menstruating, she releases this substance into the air – and this stimulates the VNO of females in surrounding (women living together in family, convents, bordellos, dormitories). Actually pheromones’ signals reach the hypothalamus leading to hormonal changes adequate to menstruating woman – it triggers changes in other women behavior and menstrual rhythm (forcing girls and/or women to menstruate at the same time).


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