Symmetry Part 3: Primer on Directional Asymmetry in Faces
Most faces show some measurable degree of asymmetry, or lack of perfect balance. Some of this asymmetry occurs more on one side of the face than the other. When it does, it is known as directional asymmetry (DA). Here are two examples:
This photo (Ercan, et.al., 2008) shows dominance in the left half of both faces. In this context, dominance means a tendency for asymmetries to be larger on one side of the face than the other, when the face is split down the midsaggital plane (dotted line). In this illustration, each solid line represents a separate DA. Ercan found left side dominance (because more features on the left side of the face were bigger/larger/further apart) and that women exhibited more DA than did men (as indicated by the number of solid lines).
The line drawing on the right (Haraguchi et.al., 2008) shows another measurement that can show asymmetries and hemifacial dominance: facial width. In Haraguchi’s sample, the right side of the face was dominant.
The reasons for facial DA are still unclear, but, sequelae to cerebral lateralization and different patterns of muscular activity (e.g., preferred chewing side, asymmetries in emotional expression, etc.) are candidate explanations. Several articles I’ve read recently have indicated that human faces tend to be right side dominant. This assertion seems premature. Ercan references studies showing both left and right side dominance and identifies several possible causes of these inconsistent results:
- Population/Cultural differences
- Age differences
- Varying measurement techniques/locations
- Variable head positioning when facial photographs are being taken
Smith (1998) even found consistent differences in hemifacial dominance in college faculty based on academic discipline. Clearly, we have yet to work out all the details associated with this phenomenon.
Ercan, I., Ozdemir, S., Etoz, A., Sigirli, D., Tubbs, R., Loukas, M., & Guney, I. (2008). Facial asymmetry in young healthy subjects evaluated by statistical shape analysis Journal of Anatomy, 213 (6), 663-669 DOI: 10.1111/j.1469-7580.2008.01002.x
Haraguchi, S., Iguchi, Y., & Takada, K. (2008). Asymmetry of the Face in Orthodontic Patients The Angle Orthodontist, 78 (3) DOI: 10.2319/022107-85.1
Smith, W. (1998). Hemispheric and Facial Asymmetry: Faces of Academe Journal of Cognitive Neuroscience, 10 (6), 663-667 DOI: 10.1162/089892998563077