Infradian rhythms: the menstrual cycle

The menstrual cycle

The menstrual cycle lasts about 28 days on average (normal cycles vary). It is controlled by hormones:

• The pituitary gland releases FSH, which makes eggs mature in the ovaries.
• Oestrogen rises as an egg matures; ovulation (release of an egg) happens around the middle of the cycle.
• After ovulation, progesterone rises to prepare the womb lining.
• If the egg is not fertilised, oestrogen and progesterone fall and the womb lining is shed (menstruation).

Hormone changes are thought to cause premenstrual mood changes in some women.

Endogenous and exogenous control

The cycle is mainly controlled internally (endogenously) by hormones, but external (exogenous) factors may also affect its timing:

• Pheromones: chemical signals released by one person that affect the body of another. Stern and McClintock (1998) tested whether pheromones from other women can change cycle length.
• Other external factors such as stress, diet and exercise can also alter cycles, which makes single causes hard to isolate.

Stern and McClintock (1998)

29 women aged 20 to 35 with regular cycles took part: 9 were odour donors and 20 recipients. Donors wore pads under their arms at different points in their cycles; the pads were treated and wiped under the recipients' noses.

• Pads from the follicular phase (before ovulation) shortened recipients' cycles by 1 to 14 days.
• Pads from ovulation lengthened cycles by 1 to 12 days.
• 68% of recipients responded.

Conclusion: pheromones can regulate the timing of ovulation.

Evaluation

• Strength: Stern and McClintock's design was controlled and used real hormone measures (the LH surge).
• Weakness: small sample (20 recipients) and 32% did not respond, so the effect is not universal.
• Weakness: many factors (stress, diet, exercise) affect cycles, so confounding variables are hard to rule out.
• Pearson's component guide says research on the menstrual cycle and SAD needs to be on human participants.

Worked example

In Stern and McClintock (1998), 68% of the 20 recipients responded. About how many women is that?

1. 68% of 20 = 0.68 x 20 = 13.6.
2. So about 14 women responded (the percentage is rounded).

Answer: About 14 women.

Check yourself

1. An infradian rhythm is one that lasts
   • longer than 24 hours
   • about 24 hours
   • less than 24 hours
   • exactly one hour
   Show answer

   longer than 24 hours. Infradian rhythms are longer than a day.

2. On average, about how many days does a menstrual cycle last?
   Show answer

   28 days. The average cycle is about 28 days.

3. Which hormone rises after ovulation to prepare the womb lining?
   • Progesterone
   • Melatonin
   • Testosterone
   • Cortisol
   Show answer

   Progesterone. Progesterone rises after ovulation.

4. In Stern and McClintock (1998), what did follicular-phase pads do to recipients' cycles?
   • Lengthened them
   • Stopped them completely
   • Had no effect on anyone
   • Shortened them
   Show answer

   Shortened them. Follicular compounds shortened cycles by 1 to 14 days.

5. How many recipients were there in Stern and McClintock (1998)?
   Show answer

   20 women. 29 women: 9 donors and 20 recipients.

6. In Stern and McClintock (1998), 68% of recipients responded to the pheromones. What percentage did not respond?
   Show answer

   32 %. 100 - 68 = 32%.

7. Explain what Stern and McClintock's study suggests about the control of the menstrual cycle.
   Show answer

   It suggests that the cycle is not only controlled internally by hormones: external chemical signals (pheromones) from other women can shorten or lengthen the cycle, acting as an exogenous influence.. Internal hormones plus external signals.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Psychology specification, Issue 5 (February 2021)
• Pearson IAL Psychology component guide: WPS02 (Issue 2, November 2024)
• University of Chicago Chronicle (19 March 1998): McClintock and Stern pheromone study

Last updated 8 November 2026. Spotted a mistake? Tell us.