Why women's menstrual periods happen at the same time when they live together

The original idea of menstrual synchronisation is that women's pheromones interact when they are close together, causing them to have their periods at the same time.

University of Chicago psychologist Martha McClintock's 1971 Nature study established menstrual synchronisation for the first time. During her undergraduate days she noticed that close friends had their periods around the same time.

To properly test the notion, she asked 135 college students living in dorms to record their period start dates three times throughout the academic year. She discovered that close-friend groups menstrual periods were substantially closer together.

The phenomenon known as "the McClintock effect" is largely regarded as the first instance of pheromones—unconscious chemical messages that alter behaviour and physiology—in humans.

While some women might experience occasional syncing, it's not caused by being close together or "catching" periods through pheromones, as previously thought. Here's the explanation:

Professor Jeffrey Schank, a psychologist at the University of California, Davis, explains that menstrual synchrony is more likely a mathematical coincidence. He suggests that when women with different cycle lengths live together, their periods might occasionally seem to align due to chance.

From an evolutionary standpoint, there's no benefit to women having babies at the same time. This suggests there's no biological reason for periods to sync up.

While some women might experience periods seemingly aligning occasionally, science suggests it's not due to any special powers or biological processes. It's simply a matter of chance and overlapping cycles.