So you get your period. And then your roommate gets hers the next day. Your other roommate says hers is supposed to come around this time of the month, too… Could periods be contagious?

The question was investigated first by psychologists in the 1970s.  Professor Martha McClintock decided to learn more about what she termed the “menstrual synchrony” of 135 female college students living in female-only dorms. She offered pheromones as one explanation—pheromones are chemicals that humans secrete in fluids like sweat that can be detected by other humans.  However, McClintock’s methods were criticized as being full of errors, and follow-up studies have failed to confirm that cycles change after women spend time in close proximity to one another.  

Not convinced by McClintock’s work on menstrual synchrony, psychologist Jeffrey Schank investigated the question.  He theorized that because women have consistent cycles of different lengths, their periods can never truly synchronize.  Rather, the different cycle lengths of different bodies mean that women drift between being in and out of sync—and noticing when their periods line up.

Academics at New York University argued that trends in feminism enhanced and exacerbated the rumor of menstrual synchrony since the 70s. Increased talk of female sexuality, attachments, and friendships may have perpetuated and popularized the menstrual synchrony myth. The researchers found that more people learned about menstrual synchrony from hearing about each other’s experiences than from journals or textbooks.

Periods aren’t contagious. But you’re not crazy–nor are you alone in thinking that periods line up. When we asked 500+ women in the Glow community if they believe that their cycles sync up with those of their roommates, 50% said “Yes!” 

Menstrual cycles do overlap from time to time, allowing us to commiserate in good company. So when you and your roommate cuddle in bed watching Frozen and eating dark chocolate to distract yourselves from the cramping, science isn’t behind it so much as coincidence. But we all know, it feels good to pretend otherwise.