Infradian Rhythms - The menstrual cycle Flashcards
What is the menstrual cycle?
The menstrual cycle occurs over an average of 28 days but there are large individual variations from 20-60 days. The endogenous pacemaker is the pituitary gland which releases the hormones FSH and LH. FSH and LH lead to the production of oestrogen and progesterone. These cause the lining of the womb to become engorged with blood and an egg to be ripened and released. If the egg is not fertilised, then the lining of the womb is shed. The whole process is then repeated.
Discuss research studies into Infradian rhythms: McClintock
McClintock - kept a record of 135 participants’ menstrual cycle as they lived in a women’s college from september to april. Their menstrual cycles synchronised. She also found that the women that spent little time with men had significantly longer cycles than those that spent more time with men. Pheromones are the most likely mechanism for both these effects. (Pheromones are biological substances that act like hormones but are released into the air rather than the bloodstream. These pheromones have no smell and are not consciously detectable but they carry messages from one individual to another of the same species).
Evaluation:
This suggests that pheromones are important in regulating the menstrual cycle. Therefore exogenous factors play a role, as well as endogenous ones. However, pheromones were not directly investigated in this study. It is possible that other shared factors (such as diet, lifestyle etc) could have influenced participants’ cycles.
Discuss research studies into infradian rhythms: Russell et al
Russell et al - applied the pheromones of one woman to a group of sexually inactive women. A swab was taken from the donor’s armpits daily, the pads were treated with alcohol to remove any bacteria and then rubbed on to the upper lips of the sexually inactive women. Some women in a control group received the same treatment but using pads that had not collected perspiration. Participants did not know which group they were in. At the end of the experiment it was found that 4/5 women in the odour group had menstrual cycles that had synchronised to within a day of the odour donor. This confirmed the role of pheromones in regulating the menstrual cycle.
Evaluation:
This study is an advancement on the McClintock study in measuring the direct impact of pheromones on synchronising women’s menstrual cycles. It employed an experimental design with high levels of control over potentially confounding variables such as sexual activity and awareness of which group they were in, or if the menstrual cycle donor. This means that a causal relationships can bee established and shows strong scientific evidence. We can say with some conviction that the presence of the pheromones caused the synchrony in the menstrual cycle. However, the sample size was small which makes it difficult to generalise beyond the study. It also offers no explanation for why one person did not respond to the donor’ pheromones.
The study is however, consistent with the explanation that the role of pheromones is to give an evolutionary advantage. Women who ovulate more often when around men are likely to have more offspring, but ovulating when there are no men around would waste resources. Also menstrual synchrony would mean that women are likely to give birth at the same time, and so could share breastfeeding and child care. there could also be potential real life applications for women who are trying to conceive who are advised to ensure plenty of male company.
Discuss research studies into infradian rhythms: Reinberg
Reinberg - a woman spent 3 months in a cave, with only dim lighting. her menstrual cycle shortened from 28 to 25.7 days during her stay in the cave and took 18 months to return to normal.
Evaluation:
This is a useful study which suggests that the menstrual cycle may be affected by levels of light. It is also consistent with other research that suggests light is an important zeitgeber affecting menstruation. For example, Reinberg also studied 600 girls and found that menarche (onset of the first period) was more likely to occur in winter. Also blind girls begin menstruation earlier. However there are limitation with using a single participant and we can not generalise to all women on the basis of this study.
