It's a classic girl-bonding scenario: While moaning to your roommate about uterine cramps, premenstrual syndrome or some other such periodic inconvenience you realize that she, unlucky girl, is having her period, too. Momentarily distracted, you take a collective step back to marvel at the wonders of human biology that have allowed your ovulation cycles to synchronize.
Though widely accepted as a fact of female life, many psychologists and anthropologists doubt the existence of such menstrual synchrony. Nearly half of the papers published on the topic find no evidence that close co-habitation draws menstrual cycles closer together. What's more, studies that do find an effect have been dogged by harsh criticisms of poor design and naive statistical analyses.
Menstrual synchrony was first demonstrated in a 1971 paper published in Nature by Martha McClintock. The University of Chicago psychologist had observed during her undergraduate days in an all-female dorm that close friends tended to get their periods at the same time.
To test the idea formally, she asked 135 college girls living in dorms to recall their period start dates at three times throughout the academic year. She found that close-friend groups had periods significantly closer together in April (later in the year) compared with October: lessening from an average of 6.4 to 4.6 days apart.
The phenomenon was dubbed "the McClintock effect" and is widely held as the first example of pheromones—unconscious chemical signals that influence behavior and physiology—among humans.
Many subsequent researchers went on to reproduce the results from McClintock's original experiment in people, rats, hamsters and chimpanzees. But a cohort of studies that found no evidence for menstrual synchrony began to grow, too.
The father–son team of Leonard and Aron Weller, both at Bar-Ilan University in Israel, conducted the most studies on humans; they looked at college dorm roommates, athletes, lesbian couples, mothers, sisters, friends and even office colleagues throughout the 1990s. Sometimes they found signs of synchrony and other times not, with no explanation why. "The answer is not clear," the elder Weller says. "At one time before we started doing our research it was sort of a truism. But if it exists it is certainly not ubiquitous."
In 1992 H. Clyde Wilson, now an emeritus professor of anthropology at the University of Missouri–Columbia re-analyzed McClintock's first experiment, along with a few others that used a similar design. He found that all had inflated the difference between period start dates at the beginning of their studies. Correcting this and other methodological errors stripped away significance from McClintock's original results, he wrote.
And McClintock's former colleague, psychologist and mathematical modeler Jeffrey Schank at the University of California, Davis, found in a highly controlled rodent pheromone study that their model of two pheromones—one that pulls ovulation forward and one that delays it—driving synchrony didn't work. "That was very disappointing to me," he says. "I really wanted those models to work out."
The insurmountable hurdle in all the studies, he says, is that women often have persistent cycles of different lengths. As such, they can never truly synchronize, just randomly phase in and out of synchrony over the months as their cycles diverge and converge.
Last year, he co-authored a study in Human Nature following 186 female Chinese students living in dorms for an entire year, the longest menstrual synchrony study yet. He saw no evidence for the phenomenon, but plenty of random overlaps that could be seen as synchrony if viewed through a shorter time window.
McClintock, however, remains resolute. Focusing on narrow definitions of precise synchrony misses the greater point, she says: whether the social environment of women can affect the timing of ovulation, not menstruation per se, no matter in which direction or what pattern. "I don't think there is any doubt that social interaction among women and body compounds from women can change the way the ovary functions," she says.