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 synchronisationUniversity 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 re…