Relationships Between Facial Expressions and Facial Shape

The relationship between facial expressions and facial shape is surprisingly understudied. We use facial videos and photographs, along with others' judgements, to investigate the interplay among facial morphology, dynamics, and perception. Facial expressions were quantified through Action Units (AUs) using the Facial Action Coding System. Facial shape was analysed using geometric morphometrics. Analysis of the relationships between 14 AUs and 13 morphological parameters revealed that women display more intense facial movements, but men exhibit a tighter link between facial movements and facial structures. Next, we used Principal Component Analysis for a broader categorisation. The principal components (PCs) of the AU data corresponded to groups of AUs, revealing specific facial expressions. Three of the expressions thus identified were the same in both sexes, while the fourth was sex specific. The PCs of the morphometric data were more mosaic in composition; however, one male and one female PC largely agreed, reflecting a similar facial-shape phenotype. In both sexes, this facial shape dimension was associated with the tendency to produce a Duchenne smile. In turn, the expression of a Duchenne smile systematically affected how people in the videos were judged by an independent panel on various attributes (e.g., generosity, trustworthiness, happiness). In men, a male-specific PC of the morphometric data corresponded to facial masculinity, as determined by Linear Discriminant Analysis. Men with more masculine facial shapes were more likely to produce lower-face anger expression. We conclude that natural facial expressions are to a certain extent related to facial morphology, especially in men.