Eyes: Fight my ignorance on Windows to the Soul? (question for artists)

Mods might think that this might belong better in IMHO or MPSIMS. Which is fine, feel free to move it. But I put it here because I’m trying to come at this from an artist’s point of view.

“They” say that the eyes are the window to the soul. Being that criminals’ identities are often hidden via stereotypical bandits’ masks or pixelation, I am prepared to accept this. I take it on faith that any human’s most distinguishing feature is his or her eyes.

But I’ve never gotten that. I guess I’m just not an eye person. I tend to identify people by their mouths. Pouty, a frowny smile, an upturned scowl, crooked teeth, a Bush smirk – to me these are the identifying features that distinguish one person from another.

So can someone please explain to me what it is about eys that is so distinguishing? Is there some mathematical or angular relationship in them that makes them such an identifying feature? Put in clear words, what distinguishes, say, GWB’s eyes from, say, Lucy Liu’s?

(Put in this forum because I’m trying to draw/paint a portrait of my niece, and I’m trying to capture the gestalt of her transition from mischievous child to lovely but silly young lady.)

In purely physical terms a lot of it is actually the skin and muscle around the eyes. A lot of eye-area facial expression comes from how open or closed they are. Like, when somebody smiles the skin around their eyes crinkles. Take a look at http://www.bbc.co.uk/science/humanbody/mind/surveys/smiles/index.shtml (it’s a quiz but while it’s fun to try to guess I think it’s useful here as an illustration) and pay attention to the eyes - how when people smile in such a way that their eyes do narrow, it makes the eyes kind of look emotionally warmer. Similarly a look of childlike innocence is associated with wide open eyes - where the entire iris is visible rather than just the middle - like this http://tomtolman.com/v-web/gallery/albums/Jan-05-Reunion/eyes.jpg -contrast the woman’s eyes in which you can see two-thirds of the iris (and it’s a great example of the smile-crinkles adding a sense of warmth to the expression) as opposed to the baby’s eyes.

Other useful things to look at are cosmetics tutorials. Cosmetic artists know how to use light and shadow to tweak the appearance of eye shape which has a lot to do with a person’s individual eye look - the bone structure around the eye.

As for the eye itself, anything it’s saying aside from colour and pupil dilation is purely ineffable to me. Which is not to say these aren’t important for capturing personality. Take a look at the way the lack of contrast between a dark brown iris and the pupil can make a brown-eyed person’s eyes look like the iris is bigger and give an over all different impression than a blue eyed person’s eyes:
http://www.juliebrodiephotography.com/images/color/BrownEyesWPCRW_2569.jpg

Anyway. Just rambling. Must get more coffee. Good luck!