I feel you, fellow chimp (Image: Brad Wilson/Getty)

Chimpanzees and humans may share the same ability to empathise with other individuals by involuntarily matching their pupil size. The mimicry only appears to work between two humans or between two chimpanzees but not between species, suggesting the signalling reinforces social bonds within species.

We already know that pupils change shape in response to a new, unfamiliar target: they tend to constrict initially and, after a fraction of a second, readjust and dilate. There’s evidence that human pupils dilate more rapidly while adjusting if their owner is interacting with another human whose pupils are also dilating. The dilation-adjustment happens more slowly if the other human’s pupils are constricting. But it is unclear when this pupil mimicry began in evolutionary terms.

To investigate, Mariska Kret at the University of Amsterdam in the Netherlands and her colleagues studied pupil mimicry in humans and, for the first time, looked for the phenomenon in chimps too. Chimps and human volunteers viewed images of pairs of chimp or human eyes for 4-second-long periods. Kret and her colleagues digitally manipulated the eye images so that after one-sixth of a second, the pupils on view started either dilating or constricting by a fixed amount, reaching their new size within a second and staying that size for the remaining 3 seconds of viewing time.


Unobtrusive binocular eye-trackers monitored and recorded whether pupil size in the viewer – whether chimp or human – changed as they gazed at the various images.

The pupil dilation in humans and chimpanzees did indeed subtly alter in response to the different images. In keeping with expectations, after initially constricting, a human’s pupils dilated more slowly when they viewed a human whose pupils were constricting. If the human on view’s pupils were dilating, the volunteer’s pupils dilated more rapidly after the initial constriction. At the end of the 4-second-long viewing period there was a 0.2 millimetre difference in the volunteers’ pupil size, on average, depending on whether the eyes in the image had dilated or constricted.

Mother phenomenon

Interestingly, the same applied to the chimpanzees: their pupils, too, dilated more slowly when they viewed a chimp with constricting pupils. The effect was a great deal more subtle in the chimps, but a statistical analysis showed the differences were nonetheless significant. The most striking effects in chimps were seen in three mothers, suggesting that motherhood increases a chimp’s subconscious awareness of pupil size.

The experiments also revealed something else: pupil dilation in humans is not altered by viewing images of chimps with either dilating or constricting pupils, and vice versa.

The results suggest that pupil mimicry might have a long evolutionary history, says Kret, because if the phenomenon is present in both humans and chimps it is possible it originally evolved in a common ancestor of the two species.

Neil Harrison at the Brighton and Sussex Medical School at the University of Sussex, UK, says that finding pupil mimicry for the first time in chimps is extremely significant, especially the accentuated effect observed in the three chimp mothers, as this suggests they are more sensitive empathetically.

All white now

The findings might even hint at a new reason for why humans evolved sclera, the whites of our eyes. “Traditionally, it’s been thought that the evolution of white sclera was driven by its enhanced ability to indicate gaze direction, and hence share attention,” says Harrison.

In a landmark paper in 2006, he showed that the pupils of human viewers tend to match those in images of people expressing sadness – but not other emotions.

Given that the human volunteers in Kret’s study were more responsive than the chimps to changes in pupil size, it might be that the whites of our eyes evolved to help us subconsciously spot those changes more readily, says Harrison.

Pupils also dilate when someone is aroused, stressed, making decisions or – obviously – in darker conditions. Kret speculates that the involuntary changing of pupil size in response to looking at someone else may be a subconscious attempt to reinforce bonding by showing you are interested in them, or share an emotional state.

To explore the phenomenon further, Kret is testing whether viewing eyes with dilated pupils increases trust in volunteers playing a game.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0104886