A new study claims that mimicry of each other's pupils might lead to increased trust.
Mariska Kret of Leiden University said that their findings showed that humans synchronise their pupil size with others and this behavior, over which we have no voluntary control, influences social decisions.
In the study, Kret and colleagues recruited 61 Dutch university students to participate in an investment game. The students were told that, for each trial, they would see a short video clip of their partner and would then have to decide whether to transfer 5 Euros or 0 Euros to that partner.
The clip was actually a manipulated image of a pair of eyes, programmed to show pupils that either dilated, constricted, or remained static over a period of 4 seconds.
The participants were told that their investment would be tripled and their partner would then choose what portion of the money (if any) to give back to the participant.
The participant had to make a quick decision about whether they should trust the partner and invest the 5 Euros, in the hope of seeing a greater return.
As expected, the results showed that participants were more likely to trust partners whose pupils had dilated, especially when the eyes indicated a happy expression.
According to the researchers, these findings suggested that group membership played an important role in how people interpret pupil signals.
They said that pupil mimicry was useful in social interactions in which extending trust and detecting untrustworthiness in others go hand in hand, and it benefits in-group interactions, survival, and prosperity.
The study is published in the journal Psychological Science.
