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Pupil size of discussion partners reflects trust

During eye contact, people tend to mirror the pupil size of the person they are conversing with. This social mechanism is related to the trust an individual has in the person they are talking to, according to research by psychologists at Leiden University. Publication in PNAS.

People often draw conclusions about the emotional state of others; what the other person believes, how he or she is feeling, and whether or not they can be trusted. The eyes, and in particular the pupils, are particularly important in assessing these characteristics. PhD candidate Eliska Prochazkova and lead researcher Mariska Kret from the  Cognitive Psychology department and the Leiden Institute for Brain and Cognition (LIBC) researched how the brain interprets changes in the pupil size of discussion partners. 'We know from previous research that mimicking pupil size is something that humans do even from the first few months of life,' Kret explains. 'Apes also do it: chimpanzees mirror the pupil size particularly of members of their own  species.’ This suggests that mirroring pupil size may be an old phenomen in evolutionary terms that promotes the empathetic bond with family and friends. 

Investing and trusting

For their research Prochazkova and Kret had a number of students play a trust game. While in an MRI scanner, participants were shown short videos  of different opposite numbers, after which they had to decide whether or not to transfer money to that person. If they did so, the amount was tripled. The recipient could then return the money to the investor. What the participants didn't know was that the pupils of their opposite number were manipulated to appear larger or smaller. The brain activity of the participants was monitored using eye tracking and an MRI scanner.

Wider pupil = greater trust

The results showed that the participants had more trust in the players whose pupils were made wider, and that they mirrored the pupil size. Prochazkova: ‘What was so intriguing was that mirroring the pupils was associated with raised activity in the theory of mind network: a specific brain network related to understanding others, trust and empathy.  This raised activity level coincided with decisions by the participants to lend the players larger sums of money. Kret adds: 'If we look someone in the eyes and their pupils change, our brain interprets this as if it is an expression of emotion.' 

Recognising social shortcomings

The research will show that mimicking pupil size plays a role in the brain areas that support healthy social research. Based on this finding, pupil size can in future be a useful benchmark for early recognition of social shortcomings, the researchers posit. ‘This is largely because physiological indications such as pupil size are probably not influenced by education, social interactions or active control,' Kret explains.  She wants to explore this in people with a social anxiety disorder or an autism spectrum disorder; these groups of individuals often have difficulty trusting other people.

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