Another one of the main theories about menstruation and synchronisation is that it’s rooted in evolution. The idea is that it was beneficial to the human race for all the women to be fertile at the same time for several reasons. Some people think this is because no particular female was fertile at a particular time, and singled out by the males as a result. Others think that it was so that prehistoric women could retreat together during menstruation and stay safe from predators.
Words: Vicky Spratt. Story via The Debrief.