Myth: Women are "in-sync"

Do women really synchronise their periods when living together?
18 July 2016

Interview with 

Kat Arney, Naked Scientists


This week Kat Arney has been finding out the truth behind a myth that keeps calendarturning up on a regular basis...

Kat -  If you're anything like me, if you're female and ever lived communally, such as in college halls or a shared house, you'll probably be aware of the idea that women living together quickly end up in synch - and I don't mean wanting a shower at the same time. I'm talking about periods - menstruation - and the idea that girls who live together will go with the flow together too.

It's actually a very old idea, linked to another myth that women's periods are controlled by the moon because the human menstrual cycle is around 28 to 30 days, roughly the same duration as the lunar cycle. Also, some animals do come into their breeding season together at certain times of the year - with the most extreme examples being mass spawnings of sea-dwelling species like corals - so why not humans?

Back in the early 70s this idea was put to the test by a researcher called Martha McClintock, who studied 135 17 to 22-year-old female students living in dorms at Wellesley College - a private women-only college near Boston in Massachusetts. Following them through the academic year, she discovered that their cycles did seem to synch up over time, eventually publishing her results in the prestigious journal Nature. This sparked a lot of interest in finding out why - the main suspects being hormones or pheromones produced by women at various points in their menstrual cycle and sensed by their room-mates.

A study in 1998, also by McClintock and also published in Nature, suggested that sniffing other women's whiffy armpits could shift cycles a few days one way or the other, suggesting a mechanism by which ladies could link up. But this was strongly criticised for ignoring the crucial point that other studies - conducted using more robust methods than McClintock's - had failed to find any evidence for menstruating in synch at all, throwing the whole underlying idea into doubt. McClintock's studies in non-human female animals, including rats, hamsters and monkeys, which had shown that these creatures also get in synch were also disproved by more careful research.

So what might be going on? Well, I don't know if you've ever done that thing where you sit in traffic with your indicator ticking, and watch the indicator light of the car in front? You'll find that after a while they seem to become synchronised, then gradually drift out again. What you're seeing isn't hormonal communication between your car and theirs, but a mathematical phenomenon - things with similar length cycles appearing to align every so often.

And when it comes to a bunch of women living together with similar but not identical menstrual cycles, it doesn't always run like clockwork - especially because the length of a woman's cycle and her period can shift by a day or more, or be even more unpredictable, and be affected by hormones, stress, diet and more. So there will be random times when every woman's period collides, just due to chance, but plenty of others when their cycles drift apart again. So it's time to put this one down as a myth. Period.


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