The two sides of spontaneity: Movement onset asymmetries in facial expressions influence social judgments

When forming basic social impressions, it is important to quickly and accurately classify facial expressions (including their spontaneity). Early studies on emotion perception, employing static pictures in the chimeric-face paradigm, demonstrated that expressions shown on the left hemi-face (LHF) were rated as more intense, compared to the right hemi-face (RHF). Interestingly, recent studies on emotion production, using high-speed video recordings, discovered an onset asymmetry (OAS) such that spontaneous expressions start earlier in the LHF, while posed expressions start in the RHF. Here, using highly controlled and dynamically developing video stimuli of avatar faces, we tested whether OASs in perceived faces influence the efficiency with which an expression is classified, as well as judgments of expression intensity, spontaneity, and trustworthiness. Videos of avatars making happy and angry expressions, with OASs of either 20 or 400 ms, were judged on several social dimensions by 68 participants. The results highlight the importance of the LHF for emotion classifications and social judgments: Expressions with earlier LHF onsets were not only judged to be more spontaneous but were also detected more quickly and accurately (a difference that was most evident for angry expressions with a briefly presented OAS, but not for happy expressions). Generally, these findings underscore how adaptive social perception relies on subtle cues in the dynamics of emotional facial expressions.