It's commonly believed that women who live together eventually experience the syncing of their monthly periods. But could it just be chance?
The theory behind the syncing of
menstrual cycles is that women's pheromones interact when they are in close
proximity, causing them to have their period at the same time. Many females buy
into it.
"I definitely think its
true," says Emma. "It would be too much of a coincidence
otherwise."
Emma, 24, lived with five girls at
university. She says that within a few months they all had their period at the
same time.
They were all buying tampons at the
same time, they were moody at the same time - and their one male housemate
really noticed it.
"It's a popular belief,"
says Alexandra Alvergne, associate professor in biocultural anthropology at the
University of Oxford.
"As humans we always like
exciting stories. We want to explain what we observe by something that is
meaningful. And the idea that what we observe is due to chance or randomness is
just not as interesting."
Alvergne has tried to review the
evidence for this phenomenon. She says the idea started with
research findings published in Nature, a scientific journal, in 1971. A
researcher called Martha McClintock studied the menstrual cycles of 135 women
in an American college.
"[McClintock] found that the
onset of the date of menstruation was more similar among friends and roommates
than among random pairings of women," says Alvergne.
Dr McClintock hypothesised that this
was because the women who were spending time together had the chance for their
pheromones to affect each other.
And why would this happen? The most
prevalent theory was that it was an evolved strategy among females to co-operate
with each other - to stop becoming a kind of harem for a single dominant man.
The idea is that if women had synchronized
cycles, they would all be fertile at the same time - so one man would not be
able to reproduce with them all.
"He can't manipulate all the
females at the same time so that's why it was believed to be a form of
co-operation between females," says Alvergne.
In the 1970s when this paper came
out, feminism was emerging as an important movement. Alvergne thinks this may
be one of the reasons the idea became so popular.
"I find that sometimes 'society
values' are hiding in hypothesis. And from a feminist point of view, the idea
that females would co-operate in the face of male domination is
attractive."
Other studies, some in humans and
some in other primates, seemed to show similar results.
But, there were also studies that
didn't find evidence of periods syncing. And people started to pick holes in
the earlier research - critics identified problems in the people chosen for the
study. The definition of when they were syncing was quite loose.
Perhaps most importantly, critics
pointed to the fact that in McClintock's 1971 findings she didn't account for
chance to explain why women's cycles were in sync.
"If I spend any period of time
with a woman who is menstruating, I will start my period," says Inez, 26.
"My womb is a huge team player. It just wants to be one of the
girls."
Not only that but, Inez, thinks some
women have wombs that prefer to play a leadership role.
"I've got a classic beta womb.
My friend Suzanne has an alpha womb. She will bring anyone on within a 10-mile
radius. She walks in menstruating, everyone is reaching for the tampons."
But the problem comes when the
theory seems to break down. Inez says she has never synced with her flatmate.
"I have known her since I was
12 and I think she is quite offended that I don't sync with her."
Alvergne is not surprised that women
observe overlap in their menstrual cycle with their friends.
"The question is - does it
happen by chance or not? By chance you would expect it to happen at least half
of the time."
Some academics recently decided to
find out if the findings of synchronicity of periods could be put down to chance. They looked at six years' worth of data
of the menstrual cycles of our close cousins, baboons.
"They proposed two
models," says Alvergne. "One was the model with the exciting
hypothesis" also known as the "evolved strategy" in which women
sync as a defence against dominant males.
"And the other model was the
boring model. Where the patterns are explained by chance."
The researchers compared how much
each model would account for the data being observed.They found that the model
assuming that patterns would appear by chance was the best model by far.
More research could be carried out
in the future that does reveal evidence that women's periods sync. But
currently many researchers are sceptical.
"Maybe actually, what we
observe is nothing more than randomness," says Alvergne.
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