A critical re-evaluation of the association between 2D:4D ratios and fluctuating asymmetry in humans

Background: Both asymmetry and the second and fourth digit ratio (2D:4D) relate to various aspects of human life history. Higher asymmetry with extreme 2D:4D ratios have suggested a link between both processes, indicating that early development determines levels of developmental instability. However, most observed associations may be biased because combinations of the digit lengths are used as both dependent and independent variables.

Aim: The present study explored the usefulness of asymmetry in digits 2 and 4 to study this association.

Subjects and methods: A combination of literature review, a small simulation study, and a study of 2D:4D ratios and asymmetry in a total of 100 young males and females is presented.

Results: Using asymmetry in digits 2 and 4 results in bias and increased type I error rates, leading to stronger associations between asymmetry and 2D:4D ratios. In spite of low sample sizes, significant associations between asymmetry measured in hands and face and 2D:4D ratios were detected for asymmetries in digits 2 and 4 only.

Conclusions: Using asymmetry in digits 2 and/or 4 causes bias. Excluding such results, there is currently little evidence of any association between asymmetry and digit ratios. Future studies should carefully select traits to investigate these correlations further.