Women who live together, share-house mates, sisters, colleagues, gradually sync up their menstrual cycles until they are all on the same schedule, supposedly through pheromones. Almost everyone has heard it, and plenty of people are sure they have seen it happen.
When researchers look properly, the syncing does not hold up. The idea took off from a 1971 study by Martha McClintock of women in a college dorm, but that study had methodological problems, and the basic maths works against it: cycles vary in length, so any two people's periods will drift in and out of overlap by chance, and our brains notice the times they coincide and forget the times they do not. Larger and more careful studies, including one that tracked hundreds of women over many cycles, have repeatedly failed to find genuine synchrony beyond what coincidence predicts. The pheromone mechanism has not been demonstrated either. So the convincing personal experience is mostly a trick of probability and selective memory; with cycles of different lengths, overlaps are simply inevitable now and then. The periods are not conspiring to align. They are just two clocks running at slightly different speeds, occasionally chiming together.