The power of pupil size in establishing trust and reciprocity

The eyes are extremely important in communication and can send a multitude of different messages. Someone’s pupil size carries significant social information and can reflect different cognitive and affective states that within a social interaction can prove to be particularly meaningful. In three studies we investigated the impact of a person’s pupil size on how others evaluate that person. In Experiment 1, participants played trust games in the role of investor. The results demonstrate that participants trusted happy compared to angry partners more, as well as those with dilating compared to constricting pupils, to whom they also assigned more positive personality traits including friendliness, attractiveness and trustworthiness. In Experiment 2, participants played the same trust game, but this time in the role of trustee, where they had to decide to reciprocate and share a financial gain with the investor or to keep it for themselves, and act selfishly. The results show that participants reciprocated more to partners with dilating compared to constricting pupils. In Experiment 3, we investigated and found preliminary evidence that these positive behavioural changes are likely to be specifically directed to the virtual partner and do not reflect a general positivity bias. To conclude, pupil size is an important social cue that others take into account when making social decisions.