Do Women Who Live Together Menstruate Together?

Good afternoon everyone!

I hope you all are having a great day so far. Now, the other day I was engaging in conversation w/ a female friend of mine and she asked me: "Why do women that live together tend to have their menstrual cycles relatively at the same time? At the moment it was asked, in my mind I was saying: "Ummm.... I dunno! perhaps they got together and planned it out." I mean seriously...How am I supposed to know? Guys don't go through those type of things. lol Anyways, through research and talking w/ a gynecologist I have an answer to the question that was posed. So Ashou and all the ladies out there this is for you.



Answer:

The idea of menstrual synchrony was first demonstrated in a 1971 paper published in Nature by Martha McClintock. She had observed during her undergraduate days in an all-female dorm that close friends tended to get their periods at the same time.

To test the idea formally, she asked 135 college girls living in dorms to recall their period start dates at three times throughout the academic year. She found that close-friend groups had periods significantly closer together in April (later in the year) compared with October: lessening from an average of 6.4 to 4.6 days apart.

The phenomenon was dubbed "the McClintock effect" and is widely held as the first example of pheromones—unconscious chemical signals that influence behavior and physiology—among humans.

Many subsequent researchers went on to reproduce the results from McClintock's original experiment in people, rats, hamsters and chimpanzees. But a cohort of studies that found no evidence for menstrual synchrony began to grow, too.

The father–son team of Leonard and Aron Weller, both at Bar-Ilan University in Israel, conducted the most studies on humans; they looked at college dorm roommates, athletes, lesbian couples, mothers, sisters, friends and even office colleagues throughout the 1990s. Sometimes they found signs of synchrony and other times not, with no explanation why. "The answer is not clear," the elder Weller says. "At one time before we started doing our research it was sort of a truism. But if it exists it is certainly not ubiquitous."

In 1992 H. Clyde Wilson, now an emeritus professor of anthropology at the University of Missouri–Columbia re-analyzed McClintock's first experiment, along with a few others that used a similar design. He found that all had inflated the difference between period start dates at the beginning of their studies. Correcting this and other methodological errors stripped away significance from McClintock's original results, he wrote.

And McClintock's former colleague, psychologist and mathematical modeler Jeffrey Schank at the University of California, Davis, found in a highly controlled rodent pheromone study that their model of two pheromones—one that pulls ovulation forward and one that delays it—driving synchrony didn't work. "That was very disappointing to me," he says. "I really wanted those models to work out."

The insurmountable hurdle in all the studies, he says, is that women often have persistent cycles of different lengths. As such, they can never truly synchronize, just randomly phase in and out of synchrony over the months as their cycles diverge and converge.

Last year, he co-authored a study in Human Nature following 186 female Chinese students living in dorms for an entire year, the longest menstrual synchrony study yet. He saw no evidence for the phenomenon, but plenty of random overlaps that could be seen as synchrony if viewed through a shorter time window.

With that being said, the most important questions in reference to the underlying mechanisms behind variation in the social effects on ovulation are: Why do some women not respond? and Why are some phases of the menstrual cycle more sensitive to external stimuli?

Until the relevant pheromones and their biochemical receptor pathways are better described, the current bulk of evidence suggests that popular notions of menstrual synchrony are more college town myth than dorm room reality.