Pupils are a rich source of social information. Although changes in pupil size are automatic and uncontrollable, they can convey interest, arousal, helpful or harmful intentions, and a variety of emotions. According to a new study published in Psychological Science, we even synchronize our pupil size with others—and doing so influences social decisions.

Mariska Kret, a psychologist now at Leiden University in the Netherlands, and her colleagues recruited 69 Dutch university students to take part in an investment game.* Each participant decided whether to transfer zero or five euros to a virtual partner after viewing a video of their eyes for four seconds. The invested money is tripled, and the receiver chooses how much to give back to the donor—so subjects had to make quick decisions about how trustworthy each virtual partner seemed.

Using an eye tracker, the investigators found that the participants' pupils tended to mimic the changes in the partners' pupils, whether they dilated, constricted or remained static. As expected, subjects were more likely to give more money to partners with dilating pupils, a well-established signal of nonthreatening intentions. The more a subject mirrored the dilating pupils of a partner, the more likely he or she was to invest—but only if they were of the same race. The Caucasian participants trusted Caucasian eyes more than Asian eyes—which suggests that group membership is important when interpreting these subtle signals.

Mimicry is common in social interactions. We establish rapport by adopting another's postures, facial expressions and even heartbeat. “In emotion research, there's a lot of focus on facial expressions,” Kret says. “Given that we spend so much time looking at each other's eyes, I think we can learn a lot more from the pupils.”