The role of structural facial asymmetry in asymmetry of peak facial expressions

Abstract

Asymmetric facial expression is generally attributed to asymmetry in movement, but structural asymmetry in the face may also affect asymmetry of expression. Asymmetry in posed expressions was measured using image-based approaches in digitised sequences of facial expression in 55 individuals, N=16 men, N=39 women. Structural asymmetry (at neutral expression) was higher in men than women and accounted for .54, .62, and .66 of the variance in asymmetry at peak expression for joy, anger, and disgust expressions, respectively. Movement asymmetry (measured by change in pixel values over time) was found, but was unrelated to peak asymmetry in joy or anger expressions over the whole face and in facial subregions relevant to the expression. Movement asymmetry was negatively related to peak asymmetry in disgust expressions. Sidedness of movement asymmetry (defined as the ratio of summed movement on the left to movement on the right) was consistent across emotions within individuals. Sidedness was found only for joy expressions, which had significantly more movement on the left. The significant role of structural asymmetry in asymmetry of emotion expression and the exploration of facial expression asymmetry have important implications for evolutionary interpretations of facial signalling and facial expressions in general.

This study is part of a larger programme of research that is ongoing in the Department of Psychiatry at the University of Pittsburgh, in collaboration with the Department of Computer Science and the Robotics Institute at Carnegie Mellon University. This study was supported in part by grants from the National Institute of Mental Health (MH 15279 and MH067976 (K. Schmidt) and MH51435 (J. Cohn). Additional support for this project was received from Office of Naval Research (HID 29-203). The authors acknowledge the contribution of Rebecca McNutt to this article. A preliminary version of these results was presented at the Tenth Annual Conference: Facial Measurement and Meaning in Rimini, Italy, September 2003.

Notes

¹This is in contrast to studies where participants were asked to “pose neutral” (Borod, Kent, Koff, Martin, & Alpert, 1988).

²Expression sequences were tracked using an automated facial feature-tracking program (Cohn, Zlochower, Lien, & Kanade, 1999; Tian, Kanade, & Cohn, 2001) to follow the inner eye corners and the superior edge of the philtrum (top centre upper lip) automatically and provide points for standardisation of images (see Figure 1 for an example).

³When outliers for structural asymmetry were removed (n=1 woman and n=2 men with values in excess of the 95% confidence interval), analyses of gender differences in structural, peak, and movement asymmetry across three emotion conditions showed the same results as the full sample. Men in the restricted sample (n=52) had higher values for structural and peak asymmetry, F(1, 50) = 6.12, p=.017, observed power=.680; F(1, 50) = 5.70, p=.021, observed power=.649, structural and peak asymmetry, respectively. There was no difference in overall movement asymmetry, F(1, 50)=.497, p=.484, observed power=.106.