Most people with a period have a story about spending so much time around a roommate or friend that they got their period at the same time. And often, the idea that being so close to someone impacts your chemistry feels good to believe in. But is it true? Can periods really sync up?

What is period syncing?

"Period syncing refers to the belief that [people] who live together or spend a lot of time together will begin menstruating at the same date," said Susan Milstein, Ph.D., sexuality educator on the medical review board of Women's Health Interactive.

You may also know period syncing as the "McClintock effect." That's because the idea of period syncing originated in 1971, when researcher Martha McClintock published the first study examining the phenomenon. This study sampled 135 cis women living together in dorms and concluded the women's periods were, in fact, syncing up. However, this study has been widely criticized because it had a small sample size and didn't control for several variables.

So, is period syncing real?

Well, it depends on who you