Women Menstruating At The Same Time

Stupid Question ™
Nov. 19, 1998
By John Ruch
© 1998

Q: Why do women who live together menstruate at the same time?
—Dara


A: The stock answer to this phenomenon, known as menstrual synchrony, is pheromones—odorless chemicals given off by some animals to provoke a reaction in others of their kind. Presumably, women pick up pheromones from each other that cause them to ovulate around the same time for some still unknown evolutionary benefit.

Martha McClintock, a Harvard student in 1971, knew that female rats can usually be stimulated to go into heat by pheromones in the urine of other females, and wondered if human synchrony had a similar cause.

In a study of her all-female dorm, McClintock found that the average difference in the onset dates of the women’s periods narrowed as time went on. The narrowing was most marked between roommates and friends—in other words, those who spent the most time together. Pheromones were postulated as responsible.

Popular folklore exaggerated McClintock’s findings into the untruth that women who live and work together always menstruate at the same time—a “fact” often observed through the lens of selective perception (remembering when it happens and ignoring contrary cases).

Meanwhile, McClintock’s study came under fire for at least one major flaw: its subjects self-reported their results. Many duplicate experiments failed to find synchrony. Others that did find synchrony failed to show that time spent together was a factor.

The statistical significance of “synchrony”—a very broad term encompassing anything from weeks to four days—was also questioned. Attempts to demonstrate a direct pheromonal affect on menstruation remained seriously flawed.

Then in March of this year, McClintock, now a professor at the University of Chicago, came out with another breakthrough study.

She and a fellow researcher took swabs from the armpits of menstruating women, then distilled them and wiped the juice under the nose of female test subjects. (Hope they were paid well.) Sixty-eight percent of these subjects had their cycles altered by one day to two weeks. (The others had no changes.)

Though still unconfirmed, this well-designed experiment seems to demonstrate the existence of human pheromones and the ability of some women to alter some other women’s menstrual cycles.

More research will have to be done to see how (and if) it works in real-life scenarios, and why some women seem immune.

Many cases of menstrual synchrony can probably be explained as selective perception, psychosomatic or coincidental conjunction of cycles disrupted by other factors. (Indeed, an incidental finding of McClintock’s March experiment was that normal cycle durations were more variable than medical textbooks report.)

But sometimes, it may just be a case of “pheromones, period.”