Holding a Steady Look at the Human Eye: Rebuttal to Critiques of the Gaze-Signalling and Cooperative Eye Hypotheses

Abstract

The Cooperative Eye Hypothesis (CEH) and the Gaze-Signalling Hypothesis (GSH) propose that the human eye--distinguished by pronounced scleral exposure and a uniformly white sclera-- evolved as a unique trait among primates to enhance eye-gaze visibility and facilitate cooperative communication. A recent review by Perea-Garcia and colleagues (2025) questioned four central premises of these hypotheses: (1) that human eye morphology is unique among primates, (2) that it is expressed consistently across individuals and populations, (3) that it improves gaze-following, and (4) that it is linked to the evolution of social cognition. Here, we revisit each claim through reevaluation of evidence and reanalysis of published data. First, we show that although some primates exhibit scleral depigmentation, humans uniquely combine this with high scleral exposure, resulting in markedly greater gaze visibility. Second, despite variation in scleral brightness, cross-species comparisons of sclera-iris-skin contrast confirm that human eyes remain distinct from those of other closely related apes. Third, experimental studies demonstrate that the white sclera confers a clear communicative advantage under ecologically relevant conditions, and that only humans consistently exploit these cues. Finally, developmental, cross-cultural, and neurocognitive evidence indicates that humans possess dedicated perceptual mechanisms for eye-gaze, consistent with its evolutionary embedding in social cognition. We conclude that while claims of human uniqueness should be moderated, the CEH and GSH remain the most plausible explanations for the evolution of human eye morphology. We also highlight key directions for future behavioral, anatomical, and genetic research.