Facial Attractiveness: The Role of Iris Ratio

Abstract

The ratio of iris width to eye width in the human eye is roughly 0.6 during infancy, falls to about 0.42 in young adulthood and middle age, and then increases again after age 50. Thus the iris-to-eye ratio (IER) is a nonlinear, probabilistic signal of age. Has natural selection shaped our judgments of facial attractiveness to be sensitive to the IER? Here we present an experiment suggesting that, for male observers, the answer is yes. On each trial, an observer viewed two young adult faces that were identical except for IER (0.48 versus 0.42), and indicated which face was more attractive. Male observers preferred the larger IER (which signals youth) in both male and female faces, suggesting that males have indeed been shaped by natural selection to be sensitive to the IER in their judgments of facial attractiveness, and to prefer IERs indicative of youth. Viewing faces upside-down did not affect this preference, indicating that IER is processed primarily as a local feature in the analysis of facial attractiveness. Females showed no preference for larger or smaller IERs, a result that invites further exploration.