The eyes are the mirrors of one’s soul and a look says more than a thousand words. There are plenty of proverbs about how intense or meaningful eye contact with another human being is. No matter how much I love them, when I look my cats Tommy and Saartje in the eyes, it still feels different and I notice something. Why is it that people have a lot of white in their eyes and animals have none or hardly any?
The classic ‘cooperative eye hypothesis’ (CEH) was the first to answer this question. People use their eyes not only to see, but also to communicate, Michael Tomasello argued in 2007, building on research from the Tokyo Institute of Technology in 2001. Because the white of the human eye (called ‘sclera’ in scientific circles) differs greatly from the color of the iris and the black of the pupil, it is easier to follow the direction and movement of another person’s gaze. This non-verbal communication option makes people better able to coordinate behavior, work together and consider what someone else might think or see. This would allow people to develop their social skills and cooperation at a high and refined level.
Animals, even other primates, have this considerably less: most animals have completely dark eyes or a colored (also pigmented) part around the pupils, where you actually only see the iris. Some animals have a little bit of visible white of the eyes and a few exceptions even quite a bit, but on a population level not as much as humans. In animals, the direction and movement of the eyes must be more hidden, so that others cannot see which prey they have in mind. This is because they live in a more competitive environment, where human society is more cooperative. The latter became possible partly because evolution gradually selected white, highly contrasting sclera in humans.
Because of our highly cooperative lifestyle, our eyes differ significantly from those in the rest of the animal kingdom. The eyes of a dog, horse and snake are completely different, but have one similarity: no or barely visible whites of the eyes. Mystery solved, you might think. At the time the CEH was created in 2007, there were scientists who argued that a lot of white in the eyes could also be a sign of good health or that the feeling that people are looking at you leads to to more altruisticgroup-oriented behavior. Tomasello and his team responded to this by stating that “these hypotheses are not mutually exclusive and that highly visible whites of the eye could have all these functions.”
Recent research published in Biological Reviews does question the classical hypothesis. The researchers do not claim that this is wrong, but that it is not empirically substantiated to substantiate the current claims. “There has been a lot of focus on eye movements, while in other studies head movement attracted more attention,” says evolutionary biologist Aurora Teuben (25), affiliated with the University of Amsterdam and co-author of the paper. “The CEH states that visible whites of the eyes make it easier to follow someone’s gaze, but the difference in reaction time between humans and other monkeys is so small that the question is how decisive this is.”
The authors do not so much have a counter theory, but argue for more research. “There has been a hyperfocus on the CEH, while there are many other possible explanations,” said Teuben. As an example, she cites research into ecological factors, such as sun exposure to the eyes and research into the context in which someone moves their eyes or head. “We have a few ideas, but more research is needed before we can say anything with certainty here. Let’s ask the question again.”
