Women who live together seem to have their periods at the same time; why is that?
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.
The theory of menstrual synchronisation
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 were substantially closer together in April (later in the year) than in October, with averages ranging from 6.4 to 4.6 days apart.
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.
Why women’s menstrual periods sync
The idea that women living together get their periods at the same time, known as menstrual synchrony, has been a popular belief for a long time. However, science tells a different story.
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:
1. It's statistics, not biology
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.
2. No biological reason
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.
