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Variability in variability: does variation in morphological and physiological traits differ between men and women?

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Area of Science:

  • Human biology
  • Evolutionary biology
  • Medical research

Background:

  • A common assumption in health and medical research is that females exhibit greater biological variability than males, often attributed to the menstrual cycle.
  • This assumption has led to a sex bias in research, creating knowledge gaps with clinical significance.
  • Conversely, evolutionary biology suggests males may be more variable, potentially due to male heterogamety.

Purpose of the Study:

  • To empirically test the competing hypotheses regarding sex differences in biological variation.
  • To investigate whether the menstrual cycle is a significant driver of greater female variation (GFV).
  • To inform health and medical research practices regarding participant variability.

Main Methods:

  • Analysis of variance in 50 morphological and physiological traits between sexes using data from the National Health and Nutrition Examination Survey (NHANES).
  • Comparison of trait variation between males and females.
  • Sub-analysis excluding women with irregular menstrual cycles to assess the cycle's impact.

Main Results:

  • Nearly half of the 50 traits showed no significant difference in variation between sexes.
  • 18 traits exhibited greater female variation (GFV), but this did not dominate human characteristics.
  • Only eight traits showed greater male variation (GMV), providing limited support for the heterogamety hypothesis. The menstrual cycle did not significantly explain GFV when accounted for.

Conclusions:

  • The assumption of greater female variability in health and medical research is not widely supported by empirical data.
  • Sex differences in variation are less common than presumed, and the menstrual cycle is not a primary driver of GFV.
  • Researchers should re-evaluate assumptions about participant variability to mitigate sex bias and improve clinical relevance.