In the eye of the beholder? Gaze perception and the external morphology of the human eye

A well-known finding from research on gaze perception in triadic gaze tasks is the overestimation of horizontal gaze directions. In general, a looker model's gaze appears to deviate more from the straight line of sight than is objectively the case. Although there is, up to now, a substantial amount of evidence for what Anstis et al. (1969) termed the overestimation effect, results vary regarding the absolute overestimation factor. Starting from the occlusion hypothesis by Anstis et al. (1969), the present study examines the influence of horizontal iris movement range, operationalized as the sclera size index on overestimation factors acquired for a sample of 40 looker models. The study rendered two main findings. First, horizontal iris movement range (sclera size index: M = 2.02, SD = 0.11, min = 1.79, max = 2.25) proved not useful for the explanation of variance in the overestimation factors (M = 1.79, SD = 0.16, min = 1.49, max = 2.24) obtained separately for each of the looker models. Second, intraclass correlations revealed that variance in perceived gaze directions between observers was roughly 10 times larger (ICC = 0.189) than variance between looker models (ICC = 0.019). The results strongly emphasize the need for larger and more diverse observer samples and may serve as a post hoc justification for using only a few or no different looker models in triadic gaze judgment tasks.