You know what happens when women live together. They start to menstruate together. Suddenly everyone craves chocolate and runs out of tampons at the same time. Not only that, but the chosen cycle often seems to belong to the most assertive woman. Well that’s biology for you – it must be pheromones or the lunar cycle or something. Evolutionary anthropologists have suggested that synchrony would prevent any one woman being monopolised by a single dominant male.

Ever since Martha McClintock, a psychologist from Harvard University, published her 1971 study of 135 female undergraduates living in a college dormitory together, it has been an accepted truth that menstrual cycles synchronise when women live together. Her study, based on an analysis of about eight cycles per woman, found that roommates and close friends saw the average number of days between the starts of their periods fall from eight or nine to five days.

A control group of randomly chosen women had cycles that remained 10 days apart from each. A study in 1999 found that 80% of women believed in the synchronising phenomenon with 70% saying that it was a pleasant experience. It’s a powerful concept after all – that the empathy of women can make their periods fall in line. But is it really true?

The solution

Well if you are one of the 80% of women who believes in synchronicity – brace yourself. It isn’t a thing. Since McClintock’s study there has been enough research with negative results to move menstrual synchronicity into urban mythology. Many studies have tried to replicate McClintock’s findings – some have succeeded, but more have not.

Criticisms of McClintock’s work include statistical errors – not controlling for chance in the results and inflating the initial differences in the onset of menstrual cycles that led to synchronicity being over stated.

A study of Dogon women in west Africa, who were segregated into menstrual huts, found no synchronicity over 763 days and no effect of the moon on periods (and these were ideal test conditions, as there was no electricity).

A study of 186 women in China who lived together for a year also found no synchronicity. But the researchers pointed out that the start of periods varied for women, and cycles were often variable, which could give the false impression of synchronicity. Menstrual cycles can vary from between 21 to 35 days. Stress, weight loss or illness will all disrupt periods. A study of 26 lesbian couples found no synchronicity but did find individual menstrual cycles varied by up to 10 days. So despite the internet refusing to let this myth die – you are the owner of your menstrual cycle and no friend, however close, can control it.