The cooperative eye

Ull mirant fixament

Most of us pay close attention to faces, especially other people’s eyes. Eyes express emotional states, but they also help us to read other people’s intentions and where their attention is focused, which is useful information to guide our social behaviour. Eye contact can be used to establish and strengthen emotional bonds, but depending on the socio-cultural context, looking directly into another person’s eyes can be very intimidating.

The tendency to look into each other’s eyes is almost innate. Looking at each other is important for the development of motor and emotional synchrony between infants and their caregivers, and can facilitate the learning of language and social skills. The tendency to avoid eye contact is an early indicator of ASD and often persists throughout life, creating significant social and occupational barriers for people on the spectrum.

Humans have complete depigmentation of the sclera and conjunctiva, producing the white of the eye, which is easily seen due to the horizontal shape of the eyelids. The cooperative eye hypothesis proposes that these features evolved because they facilitated gaze-based communication, which would have promoted cooperation by facilitating interpersonal coordination. This hypothesis is based on the idea that both our white sclera and the complexity of our social cognition are unique among primates. However, research in recent decades has shown great variability in eye pigmentation in other primate species (for example, most bonobos and Sumatran orangutans have light-coloured scleras). Furthermore, the centrality of the white of the eye in identifying gaze direction has been questioned: many chimpanzees have pale amber irises contrasting with their typically black sclera (with the same level of contrast as humans and bonobos), but with the reverse pattern.

On the other hand, many primate species can follow the gaze direction of other individuals to obtain information about the location of objects or subjects, and in some species it has been suggested that they may use this information in complex forms of social cognition, such as visual perspective taking or deception, although these abilities are still under debate. Some primates use eye gaze to convey information about their intentions or to call for help from conspecifics when threatened by other members of the group. They also look into each other’s eyes as a way of expressing and reinforcing sexual interest and bonding. When a mating pair looks into each other’s eyes during mating, copulation lasts longer and post-mating affectionate behaviour increases. In this context, it has been proposed that face-to-face sex, which occurs in humans, bonobos, and orangutans, was selected for because it facilitates eye contact during intercourse.

An alternative hypothesis to the cooperative eye is that the characteristics of the human eye are the result of sexual selection. Manipulated image experiments would support this hypothesis. In these studies, people with white sclera are judged to be younger, healthier, more attractive, and more trustworthy than those with darker sclera. The colour of the sclera could be an honest sign: it can reveal health problems (yellow from excess bilirubin, indicating a liver problem) or bad habits (redness from smoking or poor sleep). Indeed, there are now many eye drops on the market that claim to restore the whiteness of the eye. A new (and profitable) business for pharmaceutical companies.

© Mètode 2023 - 117. The legacy of the dinosaurs - Volume 2 (2023)

Serra Húnter Associate Professor of Psychobiology of the Department of Experimental Medicine of the University of Lleida (Spain) and member of the «Brain Development and Evolution» Research Group of the Biomedical Research Institute of Lleida (IRBLleida). Her research career focuses on the study of animal behaviour and its biological basis, especially on the relationships between brain, behaviour and evolution.