Do women who live together get their periods together, or is it a myth?

Women don’t always cycle at the same frequency, so irregularity can sometimes lead to menstruation occurring together, “but it’s statistical,” Schank says. “Cycles vary in length, and all this variability will lead to convergence and divergence.” Not all women menstruate on a regular schedule; while many have a 28-day cycle, others can have shorter, longer or very irregular ones.

The belief that women sharing space had their periods together originated in a 1971 paper by psychologist Martha K. McClintock, who studied 135 women in a college dormitory and concluded that social interaction has a strong effect on the menstrual cycle, probably because of something physiological. Support for this so-called McClintock effect has persisted, despite many later studies that were inconsistent and failed to prove her hypothesis and challenged her methodology.

McClintock, professor emerita of psychology at the University of Chicago, says the science has changed since her original paper was published. She now believes that pheromones secreted from the armpits of women in close quarters changes the timing of ovulation, prompting simultaneous ovulation — not menstruation.“I am aware of all this focus on mathematics, but we’ve gone beyond that,” she says. “In the phrase ‘menstrual synchrony,’ scratch out ‘menstrual’ and put in ‘ovulation’.“

Social psychologist Leonard Weller, professor emeritus in the department of sociology and anthropology at Bar-Ilan University in Israel, conducted a series of small studies — about 15 by his count — in the 1990s with his son, Aron Weller, professor of psychology at Bar-Ilan, on menstrual synchrony. They found that sometimes women were in sync — and sometimes they weren’t. He agrees with Schank that the alignment in cycles was a mathematical coincidence.

“The majority opinion is that it is a mathematical coincidence,” Leonard Weller says. “If you plot the onset for each of two women over a period of time, you will probably find they will converge as well as become disparate, having nothing to do with pheromone influence. Also, assuming the normal menstrual cycle lasts about five days, two women will have some overlap in the timing of their cycles. This has nothing to do with synchrony.”

Noha Ahmed, an obstetrician-gynecology resident physician in D.C., speaking on behalf of the American College of Obstetricians and Gynecologists, remembers hearing about menstrual synchrony when she was in college, but says existing studies have been too varied and conducted in too small samples to support the idea.

“It can be hard to say why so many believe this very common misconception,” she says. “If these individuals are all living together, they may experience overlap in the timing of their periods. I imagine that this is where the misconception stems from.”

Menstrual synchrony can provide a form of gendered solidarity for some — a sense of sisterhood — for an experience traditionally regarded as shameful and stigmatizing, says Breanne Fahs, professor of women and gender studies at Arizona State University and author of two studies that explore the sociological implications of many women’s belief in menstrual synchrony.

“For some women, there’s also the notion that menstrual synchrony is somehow magical, and they become very upset when you tell them it probably isn’t true,” Fahs says.