How facial reactions to others’ expressions are influenced by perceived social traits

Humans spontaneously mimic facial expressions. Facial mimicry is not a mere motor copy but reflects social context in service of social goals like facilitating affiliation. The study of social context has focused on face-external settings like interpersonal relations. The impact of face-internal social context on facial mimicry has, surprisingly, not been investigated yet. Here, we test if social information carried by facial shape, specifically trustworthiness and dominance, influences facial mimicry. We used virtual avatar faces from an established database known to be perceived as strongly (non-)dominant or (non-)trustworthy and added smiling, frowning, and neutral expressions. We video-recorded facial reactions and asked participants to self-report on arousal, valence, and dominance they experienced. In a second task, the same participants were asked to deliberately imitate facial expressions. We show that self-reported feelings differ in arousal, valence, and dominance by expression, and that valence is affected by dominance and trustworthiness traits. Both social traits also affect the deliberate imitation of the expression, in particular movements around the eyes. However, we did not find any significant impact of any facial social trait on spontaneous facial expressions. Thus, our results show that, despite the spatial overlap of facial social traits and expression signals, facial mimicry circuits may be immune to the face-internal social context in the absence of an external social context.