Menstrual [men-stroo-uhl] synchrony [sing-kruh-nee], also known as the McClintock Effect, or the Wellesley Effect is a phenomenon reported in 1971 wherein the menstrual cycles of women who lived together (such as in homes, prisons, convents, bordellos, dormitories, or barracks) reportedly became synchronized over time. The existence of menstrual synchrony has not been definitively established, and studies investigating it have been controversial.
The phenomenon of menstrual synchrony also addresses the larger question of whether or not humans have and can perceive pheromones, or utilize chemosignaling. Psychologist Martha McClintock was the first scientist to do a study on menstrual synchrony, reporting her findings in ‘Nature’ in 1971.
Another study by McClintock and Stern in 1998 investigated the possible existence of pheromone release/perception by human females as the possible mechanism for manipulation of the human menstrual cycle, the idea being that humans produced compounds that regulate neuroendocrine mechanisms in others, without the other party being aware of this event or consciously detecting any odor. The study was conducted by collecting compounds from underarms of donor women at prescribed phases during their menstrual cycles, and applying the compounds daily under the noses of recipient women. The study found that odorless compounds collected from women during the late follicular phase of their menstrual cycles triggered hormonal events that shortened the menstrual cycles of the recipient women, and that odorless compounds collected from women during the time of ovulation triggered a hormonal event in the recipient women that lengthened their menstrual cycles. McClintock and Stern asserted that these findings ‘proved the existence of human pheromones’ as well as illustrated manipulation of the human menstrual cycle.
Human pheromones and chemosignals have also been studied in different contexts with regard to manipulation of the human menstrual cycle. Jacobs et al. examined the effects of natural compounds taken from breastfeeding women and their infants on the cycle length and cycle variance of recipient women. Underarm and breast secretions were taken from women who were exclusively breastfeeding and had not yet resumed menstruating. Recipient women met a set of selection criteria and were blind to the study aims and conditions. During the study, recipient women were exposed daily to the collected compounds, and the resulting effects on their menstrual cycles were tabulated. Exposure to compounds from breast feeding women and their infants increased variance in average cycle length among the women, and disrupted the cycles’ regulation. It also enhanced individual differences in cycle length and maintained the type of cycle length that a particular individual had at the time of exposure.
McClintock also conducted a 1978 study of menstrual synchrony in rats. This study found that the menstrual cycles of female rats living in groups of five were more regular than those of rats housed singly. Social interaction, and more importantly a shared air supply that allowed for olfactory communication but prohibited auditory cues, enhanced the regularity of the rats’ cycles and coordinated their estrous phases after two or three cycles, resulting in synchronization. It is not known whether this synchrony and enhancement is the result of signals from a ‘leader’ within the female group or if it is due to the interaction of several equal signals from each group member. This observation of menstrual synchrony in rodents is not the same as the Whitten Effect (when male pheromones stimulate synchronous estrus in a female population) because it was the result of the continuous interactions of ongoing cycles within a female group, rather than the result of an exposure to a single external stimulus.
A different study by Weller and Weller experimented with 20 lesbian couples had results which showed that more than half of the couples tested had the same synchronization within a two day period of each other. However, Trevathana et al. conducted a study of 29 lesbian couples that showed no evidence of synchrony, and tentative evidence towards divergence of menstrual cycles. Additionally, if all women had an average-length menstrual cycle of 28 days duration, the maximum time between two women’s onsets would be 14 days and the minimum time between onsets would be zero days (synchronization). On average, the difference would be seven days, and (in small groups) half the time would be less (if one assumes there is no McClintock effect). McClintock observed a five day difference in her 1971 study and some have suggested this could have been a random occurrence.
The interaction of theorized menstrual synchrony with differing cycle lengths has not been explained. Two women with cycle lengths that differed by two days might initially begin menstruating on the same day, but the next month would be two days apart, the month after that four days, and so on. No studies have claimed to show that the McClintock effect causes women with historical cycles of different lengths to synchronize. Methodological errors have also been proposed. H. Clyde Wilson of the University of Missouri analyzed the research and data collection methods McClintock and others used in their studies. He found significant errors in the researchers’ mathematical calculations and data collection as well as an error in how the researchers defined synchrony. Wilson’s clinical research and his critical reviews of existing research, including the suggestion that pheromones can trigger synchrony in humans, demonstrated that when the studies are corrected for such errors, the evidence for menstrual synchrony disappears.
The phenomena could serve several purposes depending on the mating and birthing cycles of a particular species. It could perhaps set the context for group mating (such as in lions) or serve to facilitate male sexual performance because of the increased presence of receptive females (such as in rats), and could also help to ensure pregnancy.
The Daily Omnivore 