The idea that periods can synchronize was first investigated in a 1970’s paper by Martha McClintock, who examined the menstrual cycles of women living together in dorms. McClintock found that after 7 months of living together, women’s periods had gone from an average of 6.5 days apart to 4.6 days apart, leading to the idea that proximity caused the periods of these women to synchronize due to some chemical signal.
However, studies since then have been largely unable to replicate these findings. McClintock's results are now largely believed to have occurred by chance or poor experimental design, with many researchers calling menstrual synchrony a methodological artifact.
While it may appear that periods are synchronizing, it is important to remember that not everyone has a 28-day cycle, as some range from 21-35 days. This variability allows synchronicity to vary and periods to occur at the same, or different times.
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