The eye provides a rich variety of information about normal and abnormal behavior and health, as noted by Scott Sleek in “Eye-Tuned” in the May/June 2014 Observer. Cues about the color of the sclera (the white of the eyes) can be added to the cited examples of iris and pupil, retina, and eye movements. Recent research that contrasted ratings of normal eye images with copies of those same images whose sclera were tinted red or yellow reveals that individuals use scleral color to make judgments about health, attractiveness, age, and emotion.
Images of individuals with reddened or yellowed sclera are rated as less healthy, less attractive, and older than those with untinted control sclera (Provine, Cabrera, & Nave-Blodgett, 2013). Although reddish and yellowish eyes have been recognized as medical symptoms since antiquity, the social and adaptive impact of such judgments has been neglected. White sclera join such traits as smooth skin and long, lustrous hair as signs of health, beauty, and reproductive fitness. Given these results, eye drops that “get the red out” are used as beauty aids.
In the emotional domain, images of individuals with reddened sclera — yellow was not examined — are rated as showing more sadness, anger, fear, and disgust and less happiness than images of individuals with normal, untinted sclera. Surprise was the only emotion about which judgments were unaffected by scleral redness (Provine, Nave-Blodgett, & Cabrera, 2013).
Scleral-color cues are unique to humans, being invisible in other primates because of their dark sclera. Only the human sclera provides the white area necessary for the display of its own color and that of the overlying, transparent conjunctiva. Red sclera are primarily the product of dilated conjunctival blood vessels, and yellow the result of the deposition of scleral lipids in aging and bilirubin in jaundice. Group members use such color cues to quickly access the emotional and health status of an individual. The evolution of white sclera and the associated color cues provide traits important to the emergence of humans as a social species.
For psychophysiologists, the conjunctiva provides a unique and easy means of directly visualizing the impact of emotion and physiological state on individual blood vessels in real time. Best of all, no special equipment is required to pursue this research — a still or video camera with a macro lens will do fine.
Few research problems offer such an attractive combination of low threshold for entry and high potential for discovery.
–Robert Provine, APS Fellow
University of Maryland, Baltimore County