The role of pheromones in the synchronisation of the human menstrual cycle

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Martha McClintock

Is there any evidence to support the oft quoted belief that women who live together have synchronised menstrual cycles? Rachel Gilby investigates the possible role of pheromones and the research that might (but only might) offer some explanation for this curious phenomenon.

The human menstrual cycle is an infradian rhythm because it lasts for more then 24 hours. Menstruation itself occurs at the end of the cycle when the lining of the womb is shed as no eggs have been fertilised since the last menstruation. As part of the human menstrual cycle, oestrogen and progesterone, as well as other hormones, control the release of eggs from the ovary, the thickening of the lining of the uterus and then the menstruation. This cycle happens repetitively throughout a woman’s life from when she hits puberty until she reaches the menopause at around the age 50 and prepares the body for the possibility of carrying a child.

Much research has been done into how, in some cases of females all living together, there can be synchronisation of their menstrual cycle. The first notable piece of work is from Martha McClintock in 1971. Whilst at university she noticed how her and her roommates seemed to have a synchronised menstrual cycle and that this could not simply be a coincidence. She then looked at the idea that pheromones, biochemical substances that are released into the air, acting like external hormones, may play a role as they are chemical messengers that pass on messages from one member of a species to another. She took a number of female friends and dorm sharers and took pheromones from them and wiped them onto the upper lip of each other. The upper lip as it was assumed this would be the best place for them to be taken in through the nostrils and go into the body in a way that mimicked how it would do naturally in real life. She then repeated this process and found that eventually, a large proportion of women in the experiment now had synchronised menstrual cycles.

In a follow up ten year longitudinal study, in 1988, McClintock and Stern then looked at how pheromones from other women effected the menstrual cycles of women with infrequent, random, ovulation. They took 29 females between the ages of 20 and 35 with a history of irregular ovulation and gathered pheromones from 9 of them at different points in their cycles. They did this to see if they could lengthen or shorten the women’s cycles by placing pheromones from different stages of other women’s cycles on their upper lip. Their study showed that 68% responded to the pheromones with their cycles being lengthened or shortened depending on where in the cycles the pheromones they were given were from.

McClintock’s studies were both in natural settings making them high in ecological validity. The fact that the women’s lives were not altered in anyway other than by the addition of the independent variable (pheromones) and the dependent variable (menstrual cycle times) is a huge benefit as if they were put in a synthetic environment/situation this could have lead to synthetic results which are not true in real life situations. Another strength of the studies is that they could be easily replicated and that her second study continued to support and give evidence for what was found in her first study: that pheromones played a role.

As with many studies, there were also some weaknesses in McClintock’s studies. Firstly no control group was used such as looking at how much of her results were affected by the pheromones and how much was effected by the placebo effect and knowledge of the other women’s cycles. Being a quite naturalistic study that was performed over a long period of time, it was also hard for other factors to be controlled such as diet and stress levels, known to have an effect on the cycles too. The samples are also something McClintock’s studies can be criticised for, only taking into account a small amount of females with a small age range. Her original sample was hugely unrepresentative of the population, with only people in that area, of that class, with that type of lifestyle and that age range were used as they were all university students living together. The small sample size also meant that each synchronised person had a huge effect on results, how can we be sure that this wasn’t due to other causes or when their cycles were originally? McClintock’s studies also did not show significant enough results to suggest that the pheromones are the complete cause of the synchronisation of the human menstrual cycle.

Strassmann suggested that the results of the work done by McClintock may have been influenced by bias. McClintock and Stern admitted that they had made adjustments slightly to the data in order to take into account the effect of nasal congestion on the results. With the small sample size, this could have resulted in a big change in results and Strassmann responded to this by saying that ‘it would be useful to know what a priori criteria were employed in making such adjustments, and whether the data analysis part of the project was done blind’, suggesting that there may have been a lack in logical facts used in making the adjustments and that it was not done objectively, instead, the adjustments were done to influence the results and help prove the hypothesis.

The study has also been criticised for not showing cause and effect, only a correlation between the pheromones and menstrual cycles. There has been little investigation into other factors and if they have more or less of an effect than the pheromones. As with things such as heart disease, could there be a number of influencing factors?  

To conclude, the research into the synchronisation of the human menstrual cycle may not be extensive enough to give a definite indication as to what causes the synchronisation. Although there are many strengths to McClintock’s work, there are also many weaknesses and the reliability of her data is also questionable.

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One thought on “The role of pheromones in the synchronisation of the human menstrual cycle

  1. A really detailed description Rachel, it goes into a lot of detail. Perhaps a mention of Russell is required, as her(?) results also support Mclintock hypothesis. Other than that though, I think it’s a great essay!

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