First discovered by psychologist and neurophysiologist Michael von Grünau and his psychology student Christina Marie Anston using human subjects in 1995,[1] the processing advantage associated with this effect is thought to derive from the importance of eye contact as a cue for social interactions.
Search asymmetry is not seen with geometric stimuli—for instance 1 x 3 rows of black and white squares—that vary in the position of the distinguishing feature, or stimuli consisting of only one schematic eye.
This effect, which has subsequently been replicated in many different search paradigms, is understood in terms of the evolutionary significance of what is considered to be one of the most critical socio-cognitive abilities of humans and other primates: gaze perception.
It has been speculated that human eye morphology evolved from a necessity for fast and accurate gaze perception for complex social interactions.
[10] 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.