10 comments January 13th, 2007at 04:43pm Posted by Eli

I’m not entirely sure why this is in the Op-Ed section, but it certainly is fascinating:

…[N]either chimpanzees nor any of the other 220 species of nonhuman primates have whites of the eyes, at least not that can be easily seen. This means that if their eyes are looking in a direction other than the one in which their heads are pointing, we can easily be fooled about what they are looking at.

Why should humans be so different? And yet we are. We can’t fool anyone. The whites of our eyes are several times larger than those of other primates, which makes it much easier to see where the eyes, as opposed to the head, are pointed. Trying to explain this trait leads us into one of the deepest and most controversial topics in the modern study of human evolution: the evolution of cooperation.

The idea is simple. Knowing what another person is looking at provides valuable information about what she is thinking and feeling, and what she might do next. Even young children know that when a person is looking at one toy and not another, she most likely prefers that toy and may reach for it. Professional poker players are often so worried about others reading their minds by reading their eyes that they wear sunglasses.


Evolutionary theory tells us that, in general, the only individuals who are around today are those whose ancestors did things that were beneficial to their own survival and reproduction. If I have eyes whose direction is especially easy to follow, it must be of some advantage to me.

If I am, in effect, advertising the direction of my eyes, I must be in a social environment full of others who are not often inclined to take advantage of this to my detriment – by, say, beating me to the food or escaping aggression before me. Indeed, I must be in a cooperative social environment in which others following the direction of my eyes somehow benefits me.

Of course, it’s possible that having large whites of the eyes serves some other purpose, like enabling me to advertise my good health to potential mates. But such an advantage would apply to other primates as well. Cooperation, on the other hand, singles out humans, as humans coordinate activities to do such things as construct buildings, create social institutions and even, paradoxically, organize armies for war.


It has been repeatedly demonstrated that all great apes, including humans, follow the gaze direction of others. But in previous studies the head and eyes were always pointed in the same direction. Only when we made the head and eyes point in different directions did we find a species difference: humans are sensitive to the direction of the eyes specifically in a way that our nearest primate relatives are not. This is the first demonstration of an actual behavioral function for humans’ uniquely visible eyes.

Why might it have been advantageous for some early humans to advertise their eye direction in a way that enabled others to determine what they were looking at more easily? One possible answer, what we have called the cooperative eye hypothesis, is that especially visible eyes made it easier to coordinate close-range collaborative activities in which discerning where the other was looking and perhaps what she was planning, benefited both participants.


We are still a long way from figuring out why humans evolved to do so many complicated things together – from building houses to creating universities to fighting wars. But the simple fact that we have evolved highly visible eyes, to which infants attune even before language, supplies at least one small piece of the puzzle of how.

This is really intriguing stuff – I had never really given much thought to why we have eyes with whites and most other animals don’t. I guess I had just automatically assumed that it was part of the physical evolution of the eye itself, with no social implications. (This could still be the case, but I don’t know of anything unique about human vision that is not shared by un-eye-whited creatures, or what the presence or absence of eye-whites would have to do with visual acuity.)

The biggest problem that I have with this hypothesis is that humans are not the only social primate; far from it. So why haven’t chimps or bonobos evolved readable eyes? Do they lack the intelligence to make use of such visual cues? Are their social groups not cooperative enough for that kind of transparency to be beneficial to the individual? Could they really be more backstabby than humans? Maybe humans have become less trustworthy since we developed eye whites…

Entry Filed under: Science


  • 1. four legs good  |  January 13th, 2007 at 7:15 pm

    We’re definitely less trustworthy. I’m not sure what this means, because I know almost nothing about the evolution of the eyeball.

  • 2. Eli  |  January 13th, 2007 at 7:35 pm

    I’m actually wondering if our eye whites will start shrinking again…

  • 3. Echidne  |  January 13th, 2007 at 8:58 pm

    I always have difficulty with what things were supposed to look like at the time when some selection took. For example, why would some humans have had white showing and others not in their eyes? And what happened to the ones who had none showing? They were somehow not mated with? Hit on the head with a rock? Kicked out of the group?

  • 4. Eli  |  January 14th, 2007 at 1:28 am

    Well, this is taking place over thousands of years, so I guess proto-humans with a little bit of white showing were just a teensy bit more likely to survive and mate due to, um, their superior nonverbal communication skills. Presumably there was a parallel selection for those who could actually *read* nonverbal signals, which makes my own existence difficult to explain.

  • 5. charley  |  January 14th, 2007 at 1:47 am

    don’t fire until you see the whites of their eyes.

    not exactly an advantage…

    this is pretty interesting in light of your lying monkeys post the other day.

    the eye is always being used in these so called anti-evolution arguments.

    only god could come up with white eyed lying monkeys, obviously.

  • 6. Eli  |  January 14th, 2007 at 2:15 am

    ‘Sfunny, I actually thought about *both* of those things, but just couldn’t figure out a way to work them in…

    Like Echidne, I have trouble actually imagining evolution in action – but I can’t believe in the alternative at all.

  • 7. Attaturk  |  January 14th, 2007 at 11:41 am

    I’m no evolutionary scientist, but one I don’t think this proposition is so cut and clear.

    There’s no doubt we have more white showing than many mammals, including other primates, but our eyes are not exactly used for the purposes there’s are. That being said, it is hardly true they have no “white”

    For example, we have shitty night-vision compared to animals with more complex and structured cones and rods. For some reason, humans long ago ceased to be nocturnal in any sense.

    The physical structure of our eye-socket is also differently set than many animals, the product of not having much need for a snout and having more gray matter up top, I would think.

  • 8. Eli  |  January 14th, 2007 at 12:12 pm

    Well, I *did* say I was skeptical.

    And the fact that they were apparently completely wrong about primates not having eye whites (and I don’t think orangs are really noted for their social groups?) doesn’t do a whole lot for their credibility…

  • 9. Kevin Hayden  |  January 14th, 2007 at 2:49 pm

    It at least explains why Giant-eyed Eli Manning gets picked off more than the QBs for the Dallas Chimpanzees.

  • 10. Eli  |  January 14th, 2007 at 2:54 pm

    Heh. I think this is an even better one.

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