Mixed matters: fluency impacts trust ratings when faces range on valence but not on motivational implications

Abstract

Facial features that resemble emotional expressions influence key social evaluations, including trust. Here, we present four experiments testing how the impact of such expressive features is qualified by their processing difficulty. We show that faces with mixed expressive features are relatively devalued, and faces with pure expressive features are relatively valued. This is especially true when participants first engage in a categorisation task that makes processing of mixed expressions difficult and pure expressions easy. Critically, we also demonstrate that the impact of categorisation fluency depends on the specific nature of the expressive features. When faces vary on valence (i.e. sad to happy), trust judgments increase with their positivity, but also depend on fluency. When faces vary on social motivation (i.e. angry to sad), trust judgments increase with their approachability, but remain impervious to disfluency. This suggests that people intelligently use fluency to make judgments on valence-relevant judgment dimensions - but not when faces can be judged using other relevant criteria, such as motivation. Overall, the findings highlight that key social impressions (like trust) are flexibly constructed from inputs related to stimulus features and processing experience.