If you’re a fan of horror movies, you’ve undoubtedly noticed that the villains’ eyes often have no visible white sclera. Are they more frightening because it isn’t “natural,” or is there another reason? A recent study published in Scientific Reports, The adaptive significance of human scleral brightness: an experimental study, Wacewicz, S., Perea-García, J.O., Lewandowski, Z. et al., may have an answer.
A white or depigmented sclera is the human exception to our primate sister species. Researchers suggest that scleral depigmentation had an adaptive function in evolution. In the study, 350 participants of diverse demographics viewed 20 facial images of primates, 10 with white and 10 with pigmented sclera, and they were instructed to look at the faces without any reference to eyes. They then rated the images on certain traits. Participants rated the images showing white sclera as healthier, younger, more attractive, less aggressive and more trustworthy, characteristics that influence choice of mate and propagation of the species. A second self-replication study showed the same results.
What’s more, the contrast between a white sclera and colored iris indicates direction of gaze. Other research has proposed that gaze tracking promoted communication and cooperation among humans in hunting and scavenging, making those tasks more effective and therefore beneficial to the group. On the other hand, primates use gaze cues to compete in foraging. A dark sclera masks direction of gaze for an advantage, although apes have been known to use gaze direction to mislead competitors. Studies also have shown that direction of gaze in humans is perceived as a predictor of behavior. In fact, a nearly universal stereotype about liars is that they avert their gaze. And professional poker players are known to wear dark glasses to hide possible cues to their intentions.
The study participants had 12 different countries of residence and ages from 19 to 74, and the results were similar across age and cultural groups. The researchers concluded that this study supports existing findings that subtle changes in scleral coloration are interpreted as cues to several parameters such as age, health or emotional state, and indicates that psychological mechanisms in the past that influenced natural selection of physical characteristics to aid survival and reproduction now play a role in our social environment. The authors of the study suggest that in humans, eyes constitute a special stimulus that is processed in unique ways. They encourage further research in the area of ocular morphology to learn more about what eyes can tell us.
• Linda Conlin
Pro to Pro Managing Editor