Why Do Human Eyes Have White Parts?
– Many Animals Hide Where They're Looking
Look closely at the eyes of a gorilla or chimpanzee at the zoo, and you will see hardly any white. Only human eyes have a wide, bright white area around the dark centre. The white acts like a signboard telling everyone around us where we are looking. Many animals hide that signboard. Humans, it seems, went the opposite way and put it on display.
In class, the teacher silently glances at the back of the room. Their face is still pointing forward, yet everyone turns around to look.
At home, your dog is staring at you. But its eyes are so dark that you can't quite tell where it is looking.
Why is it that only a human gaze can be read so clearly? The answer lies in the white around the dark centre of the eye.
Just two reasons
When the dark part moves just a little, the white on its left and right changes in width. Thanks to this sharp contrast between white and dark, you can tell where someone is looking even from far away.
If prey or a rival notices where you are looking, you lose your edge. That is why in many primates the part that would be white is dark, and hard to see.
In other words, the white of the eye is built to show the gaze. So why did humans end up with eyes that give away their intentions? Let's go through it step by step.
How far away can a gaze be read thanks to the white?
The eyeball is a sphere about 2.4 cm across. The white is the colour of the tough outer layer (the sclera) that covers it. In humans this layer is white, and a wide area of it shows through the gap between the eyelids.
Try moving the slider in Figure 1. When the eyeball turns a little, the dark iris shifts sideways by a few millimetres. In the human eye on the left, the white on each side changes in width, so the shift is easy to spot. In the eye on the right, the surrounding area is close in colour to the iris. Even with the same movement, the edge looks blurred and is hard to make out.
Run the numbers, and when the eyeball turns 10 degrees, the iris shifts by about 2 millimetres. For a person with 20/20 vision (a decimal acuity of 1.0), that works out as readable from about 7 metres away. In real life this is only a rough guide, since it changes with lighting and the angle of the face. Even so, this distance is reached only because of the strong contrast between white and dark.
Why do other animals hide their gaze?
In 1997, the Japanese researchers Hiromi Kobayashi and Shiro Kohshima compared photographs of the eyes of 88 primate species. Humans were the only species whose sclera had no colouring at all. The reports also say that humans had the greatest eye width and the largest amount of visible white.
In most other primates, the part that would be white was dark, close to the colour of the iris and the facial skin. This makes it hard to read from outside which way the eyeball is turned.
A gaze gives away your next move. A predator whose gaze is noticed lets its prey escape. In some animals' groups, a fixed stare is a signal that means "I'm picking a fight." In nature, there are many situations where hiding your gaze helps you survive.
So why are humans the opposite? A leading idea is the "cooperative eye hypothesis". In a 2007 study, babies around one year old followed another person's gaze even when that person moved only their eyes and not their head. Great apes such as chimpanzees, on the other hand, were reported to follow mainly the direction of the head. By showing each other where we look, people may have been able to say "look at that" and work together.
Most dogs have eyes in which the white is hard to see. Even so, studies suggest that dogs that have lived with people for a long time are good at picking up the direction a person points with their eyes. People with eyes that show, and dogs that developed the skill to read them: it is interesting how well they fit together.
Summary
The white of the eye makes the position of the dark iris stand out against a bright background, so that the direction of the gaze can be seen from far away. Many primates have dark sclera and hide their gaze. Humans alone are thought to have taken the path of showing their gaze to each other and cooperating.
The white of the eye is a signboard for handing your gaze to others.
Humans gave up "hiding eyes" and worked together with "showing eyes."
For why animal eyes glow at night, see "Why do animals' eyes glow in the dark?". For why human eyes can't see ultraviolet light, see "Why can't human eyes see ultraviolet light?".
- Ask a family member or friend to keep their face pointing straight ahead and look to the right or left using only their eyes.
- Step back to 1 metre, 3 metres and 5 metres, and guess which way they are looking.
- Next, have them wear sunglasses, or darken the room, and do the same again. Compare how the distance at which you guess correctly changes when the white is harder to see.
At the zoo, take a good look at the eyes of gorillas and orangutans too. The edge between the iris and its surroundings should be much harder to see than in humans.
For those who want to know more – terms, formulas and links to textbooksEach part is labelled with its level, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school biology and Mathematics I
- High school+Advanced high-school material, or textbook sidebars
- UniversityUniversity-level topics not taught in high school (comparative morphology, developmental psychology)
- ResearchQuestions researchers are still working on, not yet taught as settled fact even at university
Middle schoolTerms: this phenomenon has names
- Sclera: the tough membrane covering the outside of the eyeball. In humans it looks white and is called "the white of the eye".
- Iris: the coloured part of what we call the dark of the eye. The hole in its centre is the pupil, which controls the amount of light.
- Gaze following: following another person's gaze with your eyes to look at the same thing. It is seen even in babies who haven't learned to talk.
Middle schoolHigh schoolChecking with a formula: how far away can the gaze be read?
Treat the eyeball as a sphere and use trigonometry to find how far the iris shifts sideways. Then work out the distance at which that shift looks larger than the smallest angle that visual acuity can resolve.
| In symbols | x ＝ R × sin θ , d ＝ x ÷ φ |
| In words | Iris shift ＝ eyeball radius × sine of the angle turned. Readable distance ＝ iris shift ÷ smallest resolvable angle |
| Where it comes from | The eyeball is a sphere that rotates about its centre, so the sideways shift of a point on its surface is set by trigonometry. The distance formula comes from the fact that, for small angles, "angle ＝ length ÷ distance". |
| Symbol | Meaning and unit |
| x | Sideways shift of the iris centre (mm) |
| R | Radius of the eyeball (mm) |
| θ | Angle the eyeball is turned |
| φ | Smallest angle that visual acuity can resolve (radians) |
| d | Distance at which the shift can be made out (mm) |
| Eyeball diameter (adult) | About 24 mm |
| Sine of 10 degrees | About 0.174 |
| Angle resolved at acuity 1.0 (1 arcminute) | About 0.00029 radians |
| Eyeball radius (mm) | 24 ÷ 2 ＝ 12 |
| Iris shift for a 10-degree turn (mm) | 12 × 0.174 ≒ 2.1 |
| Distance at which that shift can be resolved (mm) | 2.1 ÷ 0.00029 ≒ 7241 |
| Convert to metres | 7241 ÷ 1000 ≒ 7.2 |
Turning the eyeball by just 10 degrees moves the iris by about 2 millimetres. If the contrast between white and dark is sharp, the calculation says it can be read from about 7 metres away. But this is an upper estimate for telling apart crisp black-and-white lines. When the white part is dark, the edge blurs and the readable distance is thought to be much shorter.
High schoolHigh school+Why the white looks white, and how easy it is to tell apart
High schoolThe sclera is a membrane of collagen fibres piled up irregularly. Because the fibres don't line up, they scatter light in every direction, and the sclera looks white. The cornea is also made of collagen, but its fibres are arranged in an orderly way, so it is transparent.
High school+How easily we can distinguish the edge of an object depends strongly on the difference in brightness (contrast). Eye tests use black on white, which is high contrast, so the value of 1 arcminute is the figure for the best conditions. If the white part is dark, the brightness difference from the iris is small, and you have to get closer to see the same shift.
UniversityComparative morphology and the cooperative eye hypothesis
Kobayashi and Kohshima (1997) measured the width-to-height ratio of the eye outline in 88 primate species, and the proportion of visible sclera (the scleral exposure index). Humans had the largest value for both, and were reported as the only species with no pigment in the sclera. They interpreted the dark sclera of many primates as "camouflage of the gaze". A 2007 study by Tomasello and colleagues showed that human infants follow gaze using eye direction as a cue, and this became known as the "cooperative eye hypothesis".
📖 Derivations and further reading: Kobayashi & Kohshima, "Unique morphology of the human eye" (Nature, 1997) / Visual acuity (Japanese Wikipedia)
ResearchWhat is still not clear
- Are white sclera really unique to humans? Later studies have reported that some chimpanzees and bonobos have relatively light sclera. The view that "only humans are special" is being revised into a matter of degree.
- Did the white of the eye evolve only for cooperation? The colour of the sclera also reveals health, through bloodshot or yellowed eyes. Other ideas, such as its being selected as a sign of good health, are also under discussion.
- Where did dogs' ability to read human gaze come from? Researchers disagree on whether it developed while living with people or was already there from the start.
So the content of this article too is "an explanation within what is known today". The story of how the white of the eye evolved is still being rewritten.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "Structure of animal bodies (sense organs)" | How the sclera, iris and pupil work |
| High school | Mathematics I "Trigonometric ratios" / Biology "Animal responses and behaviour" | Calculating the iris shift; gaze and behaviour |
| High school+ | Vision and contrast | Why a gaze is hard to read when the white is dark |
| University | Comparative morphology, developmental psychology | Comparison of 88 species; infant gaze following |
| Research | Human evolution, ethology | Debate over the origin of the white of the eye |
| – | Link to daily life | Why sunglasses make expressions and intentions harder to read |
- Kobayashi H., Kohshima S. "Unique morphology of the human eye." Nature 387, 767–768 (1997)
- Tomasello M. et al. "Reliance on head versus eyes in the gaze following of great apes and human infants: the cooperative eye hypothesis." Journal of Human Evolution 52 (2007)
- Wikipedia (Japanese), "Sclera" (強膜)
- Wikipedia (Japanese), "Visual acuity" (視力)
*This article is a general-audience science explainer. The figures given are rough guides meant to help you understand the mechanism. Several theories exist about the evolution of the white of the eye, and this article only introduces representative studies. If bloodshot or yellowed eyes persist, consult an eye doctor or other medical professional.