Humans and chimps both involuntarily mimic pupil dilation in others – but only if those others are members of the same species

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 th