Menstrual-cycle variability and measurement: further cause for doubt

Area of Science:

• Human Physiology
• Biological Rhythms
• Statistical Modeling

Background:

• Menstrual synchrony, the purported alignment of menstrual cycles between individuals, has been a subject of scientific interest.
• Previous methods, such as Weller and Weller's (1990s) preferred last month only approach, have been used to assess this phenomenon.
• However, the validity of these methods, particularly under conditions of natural cycle variability, requires critical examination.

Purpose of the Study:

• To critically evaluate the assumptions and accuracy of the Weller and Weller preferred last month only method for measuring menstrual synchrony.
• To investigate the impact of within-woman and between-women menstrual cycle variability on the detection of synchrony.
• To determine if the Weller and Weller method is biased towards finding synchrony when it does not truly exist.

Main Methods:

• Distinguishing between within-woman (intrapersonal) and between-women (interpersonal) menstrual cycle variability.
• Utilizing computer simulations to test the assumptions of the Weller and Weller measurement model under varying degrees of cycle variability.
• Analyzing the potential for entrainment processes when within-woman cycle variability is present.

Main Results:

• The Weller and Weller method's assumptions are biased, leading to a false detection of synchrony, particularly when cycle variability is high.
• The degree of this methodological error increases proportionally with the extent of menstrual cycle variability.
• The simulated errors quantitatively explain the skewed data distributions observed in previous studies using this method.

Conclusions:

• Menstrual synchrony is statistically improbable when significant within-woman or between-women cycle variability exists.
• The Weller and Weller preferred last month only method is unreliable and prone to producing artifactual results.
• Observed instances of synchrony may represent methodological artifacts rather than genuine biological phenomena, necessitating the development of more robust measurement techniques.