Any woman who's lived with sisters or in a dorm or in an urban starter apartment is familiar with the concept behind the McClintock Effect, the hypothesis that menstruating women secrete pheromones that subtly alter the cycle of other women in close proximity. Many scientists dispute this claim (as well as the existence of pheromones), says Patricia Barnes-Svarney
, a science writer who's published more than two dozen books, including the recent Why Do Women Crave More Sex in the Summer?: 112 Questions That Women Keep Asking—and That Keep Everyone Else Guessing
. She says that when researchers from the University of California decided to resolve the matter once and for all by asking 186 Chinese students to track their period for one year, they found no true menstrual synchronicity. So, you might be wondering, why did you always seem to have your period at the same time as one of your roommates? Barnes-Svarney explains that most women's periods last five to seven days, which is a sizable chunk of time. The potential for random overlap is very high—especially among groups.