best friends period synchronised
Synchronising periods is probably mathematically impossible istock

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 room mates 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.

Since then, so-called "socially mediated synchrony" has been intensely studied in various groups of women, such as room mates, 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). 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.

Casting serious doubts


However, there is now accumulating evidence that casts serious doubt on the existence of the phenomenon. First, the original 1971 study was criticised on methodological grounds. Second, a number of studies with both human groups and non-human species failed to replicate the initial findings, with at least as many studies reporting positive results as studies reporting negative ones.

Mathematical analyses have also revealed that some degree of synchrony is to be expected given the shifts in female reproductive condition over time, and that no adaptive process needs to be invoked to explain what is observed. In other words, synchrony or the overlap of cycles between females is best explained by chance.

A number of critics have pointed out constraints on the very idea of the evolution of synchrony – for instance, studies have documented the significant variability in cycle length among and within women, which can make the evolution of synchronisation a "mathematical impossibility". One in-depth analysis that looked at the distribution of menstrual cycles of women living in a pre-industrial society revealed that much of the variability in the onset and length of menstrual cycles instead depended on the idiosyncracies of women's lives, such as the timing of failed pregnancy, energy balance and psychological stress.

The hypothesis that synchronisation of menstrual or oestrus (being "on heat" in the case of many non-human primates) cycles is an adaptive process can be appealing as it suggests that evolution favoured females who cooperated in the face of male sexual domination. However, as disappointing as it may be, it seems that there is now overwhelming evidence to suggest that menstrual synchrony in humans is no more than a methodological artefact from one study that has since turned into an urban myth.

Alexandra Alvergne, Associate Professor in Biocultural Anthropology, University of Oxford

This article was originally published on The Conversation. Read the original article.