Cooperative eye hypothesis

The cooperative eye hypothesis is a proposed explanation for the appearance of the human eye. It suggests that the eye's distinctive visible characteristics evolved to make it easier for humans to follow another's gaze while communicating or while working together on tasks.

Unlike other primates, human beings have eyes with a distinct colour contrast between the white sclera, the coloured iris, and the black pupil. This is due to a lack of pigment in the sclera. Other primates have pigmented sclera that are brown or dark in colour. There is also a higher contrast between human skin, sclera, and irises. Human eyes are also larger in proportion to body size, and are longer horizontally. Among primates, humans are the only ones where the outline of the eye and the position of the iris can be clearly seen.

The cooperative eye hypothesis was first proposed by H. Kobayashi and S. Khoshima in 2002 and was subsequently tested by Michael Tomasello and others at the Max Planck Institute for Evolutionary Anthropology in Germany. Researchers examined the effect of head and eye movement on changing gaze direction in humans and great apes. A human experimenter, observed by either a human infant, a gorilla, a bonobo, or a chimpanzee, did one of four actions:

The apes were most likely to follow the gaze of the experimenter when only his head moved. The infants followed the gaze more often when only the eyes moved.

The results suggest that humans depend more on eye movements than head movements when trying to follow the gaze of another. Anthropologists not involved in the study have called the hypothesis plausible, noting that "human infants and children both infer cooperative intentions in others and display cooperative intentions themselves."

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