Do the menstrual cycles of women who live together in close proximity sync up over time? The idea is comforting. But it seems like the legendary McClintock-effect is a coincidence.
Psychologist Martha McClintock of the University of Chicago first described this topic in 1971. Women who begin to live together in close proximity – whether this is in a convent, prison or student house – experience their menstrual cycle onsets becoming closer together in time than previously.
For her research, McClintock asked 135 female students to write down the first day of each menstruation for a period of six months. The result? The periods of students who had intensive contact with each other, such as friends or roommates, began to synchronise. The researcher thought this was due to pheromones (odourless substances that occur in sweat and blood and convey signals between species). The students’ pheromones would influence the time of ovulation. This was deemed proven by a smell experiment. Some women were regularly exposed to cotton pads soaked in sweat of other women. After a while, their cycle synchronised with the menstrual cycle of the ‘sweat donor’.
A total coincidence?
Later studies about menstrual synchrony – which is still known as the McClintock-effect – came up with varying results. Sometimes the periods synced up, other times there was no synchrony at all. In 1993, the phenomena was investigated in 29 cohabiting lesbian couples. This study revealed there was no evidence for menstrual synchrony. Also research among the Dogon woman in Mali (west Africa) failed to find prove for the McClintock-effect.
Ultimately, research by American psychologist Jeff Schank, associate professor of Psychology at the University of California Davis, and the Chinese Zhengwei Yang of the North Sichuan Medical College in Nanchong, proved McClintock’s findings wrong. Schank and Yang collected data on menstrual cycles of 186 women for over a year and found no evidence for menstrual synchrony at all. Also they discovered some methodological mistakes in McClintock’s studies. For example that she knew which women were given sweat pads on their upper lip, as the sweat belonged to McClintock herself. According to Schank, from a mathematical point of view it’s impossible that women’s cycles synchronise. A woman’s cycle has varying lengths and patterns and this variation has near to no regularity. His conclusion about menstrual synchrony: it’s at the level of chance, so a total coincidence.
Also recent research among users of menstrual-app Clue – working together with dr. Alexandra Alvergne from the University of Oxford – indicates that syncing is a myth. The small study (data was analysed of 360 pairs of women who had at least three cycles over a similar time period) even suggested the opposite. According to the results, cycles from women who live together or spend a lot of time together, are actually more likely to diverge over time. Currently Clue has collected data of 1 billion menstrual cycles; the company is planning to do further research on this topic in the future.