Infradian rhythms - Biopsychology - AQA A Level psychology

Biological rhythms

Regular cycles in biological or physiological activity that repeat over time.

Infradian rhythms

Biological rhythms that last longer than 24 hours, e.g., the menstrual cycle or seasonal affective disorder (SAD).

Infradian means

occurs more than 24 hours

Menstrual cycle

An infradian rhythm in females lasting around 28 days, involving changes in fertility and hormone levels.

Duration of menstrual cycle

Typically 28 days, but can vary from 24 to 35 days.

Hormonal regulation of the menstrual cycle

Controlled by the hormones oestrogen and progesterone, which regulate ovulation and preparation of the uterus.

Endogenous pacemakers in the menstrual cycle

Hormone fluctuations act as internal body clocks regulating fertility cycles.

Exogenous zeitgebers in the menstrual cycle

External cues such as pheromones can influence the timing of the cycle.

McClintock (1971) study

Found that women living in close proximity (university dorms) reported menstrual cycle synchronisation, suggesting pheromonal influence.

Russell et al. (1980)

Supported pheromonal effects, suggesting women may influence each other's cycles through chemical signals.

McClintock (1998) pheromone study

Collected pheromones from donor women's armpits and applied them to other women's upper lips; found that menstrual cycles shortened or lengthened to match donor cycle stage.

Conclusion of McClintock (1998)

Menstrual cycle can be influenced by pheromones acting as exogenous zeitgebers, demonstrating external modulation of infradian rhythms.

Criticism of menstrual synchrony research

Findings are inconsistent and often fail to replicate (e.g., Trevathan et al., 1993 found no synchrony even in cohabiting lesbian couples).

Alternative explanation

Apparent synchronisation may occur by chance due to natural variation in individual cycle lengths.

Evolutionary explanation (support)

Menstrual synchrony may have evolved to prevent one male dominating reproduction, increasing genetic diversity.

Evolutionary explanation (against)

Synchronisation could be maladaptive by increasing competition among females for the fittest male.

SAD (Seasonal Affective Disorder)

An infradian rhythm with yearly cycles of low mood in winter, linked to reduced light and melatonin regulation.

Moon cycle myth

Menstrual cycles were once thought to follow lunar phases (~29.5 days), but modern data show no correlation.

Chimpanzee comparison

Our close relatives (chimpanzees) have a 35-day cycle, showing that menstrual synchrony is not evolutionarily universal.

Evaluation: biological vs external influences

Menstrual rhythm shows interaction between endogenous hormonal control and exogenous cues (pheromones, environment).

Evaluation: methodological issues

Pheromone studies rely on small samples, self-reports, and poor control of extraneous variables.

Key concept summary

Infradian rhythms occur less than once per day, ultradian rhythms occur more than once per day, both influenced by internal and external factors.