The Answer: The concept of menstrual synchrony has been around since at least 1971, when a Harvard researcher named Martha McClintock published an article in “Nature” showing that women living in the same dorm saw their periods inching two days closer over the course of a semester. In later studies, McClintock found that odorless compounds from the armpits of women in various stages of their cycles could affect the cycle length of other women. And it doesn’t require proximity: Pumping these chemicals into the air synced the cycles of rats housed separately.
So if you can smell your officemate’s lunch over the cubicle wall, you could be close enough to hop on her menstrual calendar.One theory suggests that menstrual synchrony is a way to force investment from a male mate. If all the women in a group are “in heat” at the same time, there’s less of a chance that a male will impregnate one, then find another mate before she is ready to reproduce again. Is this happening in your office? No. (And if it is, you should probably call H.R.) But it’s an explanation for why period syncing might be a thing in the first place.
However, there are detractors who say it actually isn’t a thing at all, and that it’s a matter of probability and perception. The average menstrual cycle is 28 days long, which means that the most the start of your period can differ from that of your friend, co-worker or roommate is 14 days, with an average of seven days. They’re just as likely to be less than a week apart, which means your respective times of the month are probably going to overlap regardless. But if McClintock’s research is right, those couple extra days could make all the difference in your social/workplace/house dynamics. Stock up on tampons and dark chocolate, just in case.