Auditory smiles trigger unconscious facial imitation.

Smiles, produced by the bilateral contraction of the zygomatic major muscles, are one of the most powerful expressions of positive affect and affiliation and also one of the earliest to develop [1]. The perception-action loop responsible for the fast and spontaneous imitation of a smile is considered a core component of social cognition [2]. In humans, social interaction is overwhelmingly vocal, and the visual cues of a smiling face co-occur with audible articulatory changes on the speaking voice [3]. Yet remarkably little is known about how such 'auditory smiles' are processed and reacted to. We have developed a voice transformation technique that selectively simulates the spectral signature of phonation with stretched lips and report here how we have used this technique to study facial reactions to smiled and non-smiled spoken sentences, finding that listeners' zygomatic muscles tracked auditory smile gestures even when they did not consciously detect them.