During face-to-face interactions, the pupils of each person’s eyes tend to synchronize or “mimic” each other. This effect, which is completely involuntary, can lead to an increased sense of trust during communication, according to a new Dutch study published in Psychological Science.
“People generally underestimate the importance of pupils, despite the fact that we look into them each day. The pupil provides a rich source of social information — we can force a smile, but we can’t force our pupils to dilate or constrict,” said psychological scientist Dr. Mariska Kret of Leiden University, lead author on the study.
“Our findings show that humans synchronize their pupil size with others and this behavior, over which we have no voluntary control, influences social decisions.”
In previous work, the researchers discovered that humans and chimpanzees synchronized their pupil size specifically with members of their own species. The researchers hypothesized that pupil mimicry might be important for the establishment of a bond of trust between two individuals.
Dilated pupils are typically perceived as a sign of safety, suggesting that mimicry of another person’s dilated pupils may lead to mutual trust. Constricted pupils, however, tend to be seen as a sign of threat, and researchers did not expect that mimicry of constricted pupils would be connected with trust.
For the study, Kret and co-researchers Drs. Agneta Fischer and Carsten De Dreu of the University of Amsterdam recruited 61 Dutch university students to play an investment game. The students were told that, for each trial, they would watch 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 pair of eyes, manipulated to show pupils that either dilated, constricted, or remained the same over a period of 4 seconds.
The students were told that their investment would be tripled and that their partner would then choose what portion of the money (if any) to give back to the participant. This scenario resulted in the student having to make a fast decision about whether they should trust the partner and invest the 5 Euros, in the hope of seeing a greater return. I
n reality, all of the partners’ choices were determined and randomly assigned by the researchers.
As expected, the findings revealed that the students were more likely to trust partners whose pupils had dilated, especially when the eyes indicated a happy expression.
Using eye-tracking technology, the researchers found that the students tended to mimic their partners’ pupils, whether they were dilating or constricting.
Most importantly, mimicking a partner’s dilating pupils was linked to the decision to invest money, but only when the partner’s eyes had a Western European appearance.
These findings suggest that group membership plays an important role in how we interpret pupil signals, note the researchers. For example, participants were more likely to trust partners with dilated pupils when they belonged to the same group (Western European descent) than when they didn’t belong to the same group (Asian descent).
“The results of the current study further confirm the important role for the human eye in what people love and fear,” the researchers write. “More specifically, pupil mimicry is 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.”