By Alexandra Alvergne, University of Oxford.
It is a popular belief that women who live together synchronise their menstrual cycles, and that it’s mediated by their pheromones, the airborne molecules that enable members of the same species to communicate non-verbally.
The idea originated in a study published in Nature in 1971, which recorded data on the onset of menstruation for 135 American college students living in a dormitory. The dorm had four corridors each with around 25 girls living in single and double rooms.
Based on the analysis of around eight cycles per woman, the study reported an increase in synchronisation (a decrease in the difference between onset dates) for roommates and among closest friends, but not among random pairings in the dormitory.
The author hypothesised that this was driven by the amount of time that women spent together, as this would allow for pheromone communication.
Watch: How much blood do you really lose during your period? (Post continues after video.)
Since then, so-called “socially mediated synchrony” has been intensely studied in various groups of women, such as roommates, co-workers, lesbian couples and women from high fertility populations, and in a number of animal species, including rats, baboons and chimpanzees.
The theory goes that synchronisation leads to females becoming sexually receptive at the same time.
There have been many evolutionary arguments for why females would synchronise the timing of sexual receptivity. These theories, reviewed here, assume that synchrony would serve to maximise the reproductive success of females (and also sometimes males).
This could be the biggest women's health myth. Period. Image: iStock
The most popular one is that it enables females to minimise the risk of being monopolised by a single dominant male, and thus make it easier to engage in polyandry.
It is true that in multi-male, multi-female groups in which both males and females mate with multiple partners, if all females are sexually receptive at the same time then it is difficult for a male to control the sexual access to a particular female at all times.
In this vein, a meta-analysis of 19 primate species found that the degree to which a dominant male would father all offspring was inversely related to the degree to which the females synchronised their cycles. In other words, a dominant male had less control over reproduction if all females were receptive at the same time. (Post continues after gallery.)