Menstrual synchrony, also known as the McClintock effect, is a process in which women who live together or in close proximity involuntarily synchronize their menstrual cycle with each other. The latest data suggests original studies validating this effect were incorrect and could not be replicated in larger populations.
The phenomenon of women's synchronized periods has been popularly referred to as the McClintock effect after Martha K. McClintock, the researcher who published an article called “Menstrual Synchrony and Suppression” in Nature magazine in 1971. The study concluded that women living together or who work closely together synchronize their periods.
McClintock commented that human females experienced the same phenomenon in dormitory life. The scientists challenged her to address the issue scientifically. McClintock took on the topic as her senior thesis at Wellesley and published her results while pursuing a Harvard graduate degree.
While McClintock's papers didn't prove a causal basis for the pheromones attributed with the effect, it did offer intriguing evidence. Further research by McClintock and others (Russell et al., 1980; Stern and McClintock, 1998; etc.) pointed to pheromones causing the menstrual synchrony.
Interestingly, other studies show that male pheromones affect women’s menstrual cycles.
Why do women's periods sync? In a phenomenon also called the dormitory effect, menstrual cycles change for women living together in dormitories, prisons, convents, and other communities. This has been purported to impact the start date and length of menses.
The same phenomenon, called the Whitten effect, has been noted in mice and guinea pigs. However, the Whitten effect is caused by male pheromones, whereas the McClintock effect involves only female pheromones.
The pheromone chemicals believed responsible are released by skin glands concentrated in the armpit. These airborne chemicals don't give off an odor but are sensed by the nose's vomeronasal organ, or Jacobson’s organ. The theory goes that women release pheromones, and these pheromones signal the hypothalamus in the brain, triggering changes in other women's menstrual cycles and leading to girls and women experiencing menstrual cycles that are closely in sync.
Although nearly 50 years of intensive investigation have passed since McClintock first published results on menstrual synchrony, there is still no conclusive evidence for the existence of this phenomenon.
A 1992 study corrected statistical errors in McClintock's original study and follow-up studies.
The study found that all the previous studies had several errors that led to biased results — the researcher concluded these biases tainted the findings, leading to higher-than-expected frequencies of menstrual synchrony. The later study concluded that there wasn't any significant menstrual synchrony impacting the onset of menses or the length of the period.
A study by Morofushi and colleagues in 2000 enlisted 64 Japanese women who lived in a dormitory. Twenty-four of the young women synchronized with their roommates in three months. Researchers concluded that the change in menstrual timing was triggered by the ability to smell 5 alpha-androstenol. Interestingly, for women who did experience synchrony, all members changed their cycles to share the same time of flow.
Two experiments and three studies conducted in the 1990s showed menstrual synchrony when subjects received applications of an axillary extract taken from donors’ underarms or after test subjects spent time together.
In 2017, in an attempt to end the debate, Oxford University researchers tested women who used a specific app — 360 pairs of women were included. Each had a close relationship with another woman over a long period of time. The app let women track and share information about their period, so researchers merely had to analyze the data.
The researchers reviewed the past three menstrual cycles among the pairs to identify whether any alignment could be detected. The findings stated that 273 of the pairs actually diverged instead of syncing. Conversely, just 79 pairs seemed to converge. Further, women who lived together did not report an increased percentage of alignment when compared with other pairs. The researcher concluded that this showed the idea of menstrual synchrony was a myth, despite the many women who still believe in it.
Why do so many women believe that their cycles are influenced by the women around them? The Oxford team concluded that it was based on the nature of the menstrual cycle itself. The average menstrual cycle lasts about 28 days — the time between when one cycle starts and the next begins. However, not everyone’s cycle is the same length, nor do their period last the same number of days. Because of this, there’s a lot of room for overlap to occur among a group of people, which may give the impression of syncing.
If you would like to conduct your own research about whether your period syncs up with coworkers, friends or rooms, you can use the Flo app to track your period and compare notes with the women closest to you.
The science seems inconclusive on whether menstrual synchrony is a true phenomenon or just a belief that has reached the status of urban legend, making it commonly accepted as fact.