Trust and cooperation are important to human evolution. We rely on neurocognitive structures that help us to quickly assess whether we are able to trust another. During interactions people tend to focus on facial features and emotional expressions. The human eye region stands out as a particular strong indicator for assessing someone’s trustworthiness. Pupil changes are unconscious, therefore reflecting honestly on a persons’ inner state. Studies have shown that these subtle pupil changes are perceived by others and influence their evaluation. It is thought that dilated pupils elicit a positive and attractive judgement of the other, where people with small pupils are conceived as distant. The current study examined whether humans decisions on trustworthiness and cooperation on their partner’s eyes and alter their assessment when the pupils change. Firstly, it was systematically examined whether participants’ pupil size adjusts (pupil mimicry) when others’ pupils are dilating or constricting or remain static. Secondly, it was examined if pupil mimicry only took place with familiar people (in-group) or also with strangers (out-group). Finally, it was examined if pupil mimicry had an influence on how trustworthiness was perceived.

Current study

Originally, 69 students of the University of Amsterdam participated, but 8 participants were excluded because of odd response patterns. The students had no history of neurological or psychiatric disorders. As visual stimuli, pictures of four men and four women of Western European descent with either a happy or an angry expression were chosen as the in-group. For the out-group, pictures of people of Asian descent were selected. All pictures were adjusted to only gray colours and only the eye region remained visible for the participants. The eyes were build up from a standard template.

The participants were seated at a 75-cm distance from a computer screen. This distance reflects personal space that is typical for personal relations and informal contacts. All participants were included in a so called ‘trust game’. This was referred to as an investment game, in which the participants had to decide to invest money in another player. This amount of money would be tripled and matched to the decision of their virtual partner. Participants were told that they would not receive feedback on the decisions of their virtual partners. The experiment used a random block design, with in- and out-group blocks, emotions (happy and angry) and dilating, constricting or static pupils. Participants heart rate, skin conductance responses and changes in pupils were recorded.

The results showed that happy partners were trusted more than angry partners. Dilation of the pupil size induced trust, especially when the eyes displayed happiness. Evidence was also found for pupil mimicry with an effect of partner pupil size, both in dilation and constriction. A link was found between partner—pupil contingent trust for in-group partners, but not for out-group partners.

Concluding it can be said that attending to other people’s pupils and synchronizing with their changes helps us to assess the trustworthiness of a person. Changes in pupils have a communicative function and are contagious. Pupil mimicry is useful in social interaction and therefore benefits survival and prosperity.