Stare-in-the-crowd effect evolved from a necessity for fast and accurate gaze perception for complex social interactions (Emery 2000). According to this model, an eye direction detector (EDD) mechanism - a neurocognitive system responsible for the rapid detection of eyes in the environment and their gaze direction – arose as a function of the geometry created by a dark circular iris relative to the white sclera (Baron-Cohen 1994). Specifically, it is contended that the increased ratio of exposed sclera in the eye outline of humans serves to enhance the velocity and acuity of gaze direction perception (Kobayashi and Kohshima 2001). Baron-Cohen, S. (1994)
Cooperative eye hypothesis 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