Menstrual cycle

• Infradian rhythms are biological processes that repeat less frequently than once every 24 hours (eg weeks / months).
• Ultradian rhythms are biological processes that repeat more frequently than once every 24 hours (eg minutes / hours).
• The menstrual cycle is an example of the infradian rhythm. It is a 28 day cycle, with a 6 day period of fertility. BUT cycles can be shorter or longer, ranging between 24 and 35 days.
• Biological aspects of the menstrual cycle:

  • Ovulation (egg release),
  • Thickening of lining,
  • Loss of lining (menstruation).

  These are regulated by hormones, oestrogen before ovulation and progesterone after ovulation. The levels of these hormones act as endogenous pacemakers, keeping the process to time and acting as an internal body clock for the cycle.
• McClintock (1971) studied 135 women living in the same dormitory, recording their period onset and duration. Results showed women appeared to synchronise their cycles with their friendship groups.
• Stern and McClintock (1998) investigated whether the cause of synchronisation was due to pheromones.

  • 20 women (with irregular periods) were given pads to wipe on their top lip each day. The pads had be taken from the armpits of 9 donor women at varying stages of their menstrual cycles.
  • Researchers found the women would either shorten or extend their cycle to match the donor, depending on the stage of when the donor's pad has been collected. This suggests synchronisation was due to the presence of a pheromone acting as an EZ (external cue for the timing of the infradian rhythm).
• Strengths of the infradian rhythms:

  • Supporting study - Menstrual cycle is also governed by EZ. Reinberg examined a woman who spent 3 months in a cave with only a small lamp to provide light. Noted that her menstrual cycle shortened from the usual 28 days to 25.7 days. This result suggests that the lack of light (an exogenous zeitgeber) in the cave affected her menstrual cycle, and therefore this demonstrates the effect of external factors on infradian rhythms.
• Strengths of infradian rhythms:

  • Supporting evidence for behaviour - Penton-Volk found that woman expressed a preference for feminine faces at the least fertile stage of the cycle, and for a more masculine face at the most fertile point. These findings indicate that women’s sexual behaviour is motivated by their infradian rhythms, highlighting the importance of studying infradian rhythms in relation to human behaviour.
• Limitations of infradian rhythms:

  • Limited explanation of synchronisation - Trevathan found cohabiting lesbian couples did not synchronise in the optimal conditions for synchronisation, suggesting that it is a limited explanation and cannot be the case for all women. Cycles vary in lengths and women can appear to synchronise due to their variability in cycles, which is a factor that was not taken into consideration in all of the studies showing positive results.
• Limitations of infradian rhythms:

  • Limited explanation - Maladaptive behaviour due to women wanting to be fertilised by the male with the best genes, and needing to compete with other women for him if they are synchronised. This could also be the potential reason they prefer more masculine faces at their high fertile stages.