Why does a womans menstrual cycle change when she is living with a group of women?
Studies have shown that groups of women living in close proximity gradually become synchronised with regards to their menstrual cycle. This was first described in 1971 by researcher Martha McClintock, now with the University of Chicago, although it was more common knowledge long before then – for example in nunneries. Synchronous menstruation has been observed among mothers, sisters, and daughters who live together, and sometimes among women who simply work together. McClintock tells of seven female lifeguards who started out one summer with widely scattered periods. Three months later they were all menstruating within four days of one another. A study of 135 residents of a women’s college dorm confirmed the effect. Most of the cycle shifting occurred within the first four months and was usually complete after seven months. The whole building didn’t synchronize, just roommates and close friends although the women were unaware of what had happened. Later research has suggested that sync
