Dilated compared to constricted pupils have been found to increase several prosocial behaviors and impressions in interaction partners, including trust decisions, as well as ratings of friendliness...Show moreDilated compared to constricted pupils have been found to increase several prosocial behaviors and impressions in interaction partners, including trust decisions, as well as ratings of friendliness and attractiveness. Dilated pupils have also been found to decrease lying in a coin-toss game where lying harmed the other person. It has been suggested that this decrease in lying might be caused by enhanced social bonding. Since increased attractiveness has also been linked to enhanced social bonding, this study investigated, whether perceived attractiveness mediates the relationship between pupil size and lying. To investigate this, a coin-toss game has been used where the participants were asked to make private predictions about the outcome of a coin-toss. They were then asked whether or not their predictions were correct. For each correct prediction, they earned some money, and their interaction partner lost some money. For each incorrect prediction, only their partner earned some money. The passive interaction partner was a confederate wearing contact lenses with either dilated or constricted pupils. After this coin-toss game, they were asked to rate their interaction partner’s attractiveness. The relationship between pupil size and lying, as well as the mediation of attractiveness, were found to be non-significant. Therefore, attractiveness does not seem to mediate the relationship between pupil size and lying. These results do not fit earlier findings. A number of possible explanations are suggested for the unexpected results. However, more research is needed to investigate these suggestions and to fully understand the effect of pupil size on lying.Show less