It’s been a common saying, that women who spend a lot of time together end up sharing not just the same space but also their periods. The origins of this might have been due to a study conducted way back in the 1970s in the highly influential journal, Nature, or just pure coincidence.

One cycle is usually 28 days long, which makes the maximum that two women could be out of sync to be 14 days. This will make an average of seven days apart, and most of the cycles tend to start even closer than the actual date. A woman’s average period typically lasts for around five days, which makes it quite possible for two women to have overlapping menses at some point or another.

Since that study in the 1970s, other studies have negated this long-standing myth with the latest study performed by Dr Alexandra Alvergne from the University of Oxford. Alvergne used a period-tracking app Clue and used the data from this application to delve into the lifestyles of 1,500 users. Questions included whether they thought that their cycles had been syncing with people close to them, what that relationship was, whether they lived together and whether they were on hormonal birth control. Results show that the cycles of these women were more likely to get out of sync over time.

The idea that periods sync up has been a long-standing myth despite other scientific studies that presented uncertainty on this theory. Dr Alvergne goes on to explain that this myth stood the test of time possibly because as humans we feel the need to explain our observations with something meaningful, rather than chalking it up to randomness.