Periods syncing, also known as menstrual synchrony or the McClintock effect, is a phenomenon where the menstrual cycles of women who spend a lot of time together tend to start around the same time. This was first scientifically proposed in 1971 by Martha McClintock, who observed that women living closely, such as dormmates, showed convergence in their menstrual cycle start dates over time. The theory suggested that pheromones—chemical signals released by the body—might influence and synchronize the cycles among women. However, more recent and rigorous studies have challenged this idea. Some research shows that menstrual synchrony can actually occur by chance due to the natural variation in cycle lengths among women. For example, since average cycle lengths differ slightly and menstruation typically lasts several days, cycles will sometimes overlap mathematically without any influence from one another. Large-scale studies have failed to find consistent evidence that periods truly sync caused by pheromones or social interaction. In summary, while the idea of periods syncing is popular and may appear to happen among women who live or spend a lot of time together, scientific evidence suggests that this is more likely due to natural cycle variability and coincidence rather than an actual biological synchronization process through pheromones or social factors.