Untrustworthiness inhibits congruent facial reactions to happy faces.

This study investigated how trustworthiness modulates facial reactions to emotional faces, specifically emotional mimicry. In order to manipulate the trustworthiness of others, we used the trust game. The participants (N=37) perceived the partners as trustworthy or untrustworthy based on whether the partners tended to behave fairly or unfairly in the game with a within-subjects design. Facial electromyograms were measured while viewing the emotional faces displayed by the partners. The results demonstrated that the zygomaticus major activity in response to happy faces was greater for the trustworthy partners than for the untrustworthy partners. The results suggest that emotional mimicry of happiness is more inhibited for those who are non-affiliative and unfavorable, such as the untrustworthy partners. On the other hand, there was no effect of trustworthiness on congruent facial reactions to sad and angry faces. However, the zygomaticus major activity for angry faces was smaller for the trustworthy partners than for the untrustworthy partners, suggesting that positive expressions to angry faces of the trustworthy partners are inhibited. Trustworthiness constructed by social exchange modulates not only emotional mimicry for happy faces but also incongruent facial reactions to angry faces.