The telltale face: Possible mechanisms behind defector and cooperator recognition revealed by emotional facial expression metrics

In this study, we investigated the role of facial cues in cooperator and defector recognition. First, a face image database was constructed from pairs of full face portraits of target subjects taken at the moment of decision‐making in a prisoner's dilemma game (PDG) and in a preceding neutral task. Image pairs with no deficiencies (n = 67) were standardized for orientation and luminance. Then, confidence in defector and cooperator recognition was tested with image rating in a different group of lay judges (n = 62). Results indicate that (1) defectors were better recognized (58% vs. 47%), (2) they looked different from cooperators (p < .01), (3) males but not females evaluated the images with a relative bias towards the cooperator category (p < .01), and (4) females were more confident in detecting defectors (p < .05). According to facial microexpression analysis, defection was strongly linked with depressed lower lips and less opened eyes. Significant correlation was found between the intensity of micromimics and the rating of images in the cooperator‐defector dimension. In summary, facial expressions can be considered as reliable indicators of momentary social dispositions in the PDG. Females may exhibit an evolutionary‐based overestimation bias to detecting social visual cues of the defector face.