The big warm smile of cheaters: lateral posing biases and emotional expressions in displaying facial trustworthiness.

Cheater detection, which is a prerequisite for the evolution of social cooperation, has been successfully simulated in laboratory settings. However, the process has not been perfect because the detection rate has usually been just above chance. The present study investigated the role of lateral posing biases and emotional expressions in displaying trustworthiness, which plays a crucial role in cheater detection. Participants ( = 30 and 28 in Experiments 1 and 2, respectively) observed facial photographs of cheaters and cooperators in an economic game and evaluated their facial expressions in terms of emotional valence and arousal. The models in the photographs had turned their left or right cheek to the camera to display their trustworthiness in the economic game. The results indicated that cheaters showing their left cheek were rated as more emotionally positive than cheaters showing their right cheek. This lateral difference was not observed for cooperators. A left cheek advantage in emotional arousal was found for both cheaters and cooperators. These results suggest that cheaters use a fake smile on the emotional side of their face (i.e., the left) to conceal their uncooperative attitude.