Why do the menstrual cycles of cohabitating women synchronize ?

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  It has long been noticed,  and it has now been scientifically confirmed, that women who cohabitate for long enough eventually have their periods at the same time.  In other words,  they synchronize.    It is reasonable to assume that this occurs under the influence of pheromones. Pheromones are airborne chemical signals that are released by an individual into the environment and which affect the physiology or behaviour of other members of the same species. The idea that humans produce pheromones has excited the imagination of scientists and the public, leading to widespread claims for their existence, which, however, has remained unproven. Stern and McClintock recently published a paper in the reputed journal  Nature   investigating whether humans produce compounds that regulate a specific neuroendocrine mechanism in other people without being consciously detected as odours (thereby fulfilling the classic definition of a pheromone).  They found that odourless compounds from the armpits of women in the late follicular phase of their menstrual cycles accelerated the preovulatory surge of luteinizing hormone of recipient women and shortened their menstrual cycles.  Axillary (underarm) compounds from the same donors which were collected later in the menstrual cycle (at ovulation) had the opposite effect: they delayed the luteinizing-hormone surge of the recipients and lengthened their menstrual cycles. By showing in a fully controlled experiment that the timing of ovulation can be manipulated, this study provides definitive evidence of human pheromones.

Certain body odors,  which such women are not aware of,  probably entrain mestruation,  such that any subgroup of women who just happen to be synchronized will have a predominant effect on the rest of the cohort,  eventually leading to full synchrony.    One could wonder why I am talking about this phenomenon in a book on differences between men’s and women’s brains.   The reason is that the brain is the central link between the input and output dimensions of the phenomenon.   Some area of a woman’s brain has to detect the pheromones in question,  and the same or a different brain area has to influence that woman’s endocrine system to make it change it’s cycle.  We have no idea what these brain centers might be,  and of course,  we know even less about the brain physiology involved.   But we have good leads from animal research on sexual attraction by pheromones.   So some of the first answers to these questions will surely come from animal research. In the meantime, we know that as with other female mammals, a man’s "smell" appears to become more important to women near ovulation. Some evidence suggests that this is related to increasing estradiol levels and their effect on olfactory acuity and specificity.   Now,  why in the world would nature want the human menstrual cycle to synchronize in cohabitating women ? A sexology researcher named Edward Miller has recently argued that synchrony is a device to make it harder for dominant men to force sex on women and incorporating them in harems. Ovulatory synchrony would make it easier for women to pair bond with men...   Of course,  he is speculating here about mechanisms dating back to very primitive times,  long before marriage became socially regimented.