If you are studying biorhythms as part of your Psychology course, you are likely to come across an intriguing piece of research by Kathleen Stern and Martha McClintock that demonstrates how menstrual cycles (an infradian rhythm) may synchronise as a result of the influence of female pheromones.
The original study in 1968 involved 29 women with a history of irregular periods. The researchers took pheromone samples for 9 other women. These samples were taken at different stages of their menstrual cycles and gathered using a cotton pad placed in their armpit. The pads were then placed on the lip of the 29 participants – having been treated with alcohol and frozen! The study found that 68% of women experienced changes to their cycle which brought them closer to the cycle of their ‘odour donor’.
A recent piece on the BBC website looked at this research, examining possible explanations as to why menstrual cycles may sync, such as being a form of co-operation between females. In our evolutionary past, it might have been a form of defence against a man forming a harem because all the women in a community would be fertile at the same time – so one man would not be able to reproduce with them all and therefore a harem would not be the best strategy for the male, working to the females’ advantage.
A recent study (Clarke et al. 2012) examined six years’ worth of data of the menstrual cycles of baboons. They concluded that the data fitted the chance model best i.e. synchronisation just occurs due to chance – inevitably in any group of women menstrual cycles many will follow the same pattern. So this suggests that the observed effect is just a chance phenomenon not an example of pheromone synchronisation.