Symmetry in the eye of the beholder

A hypothesis about why symmetry along the vertical axis works as a yardstick of beauty



The relation between beauty and facial symmetry is well recognized. People judge faces as “beautiful” if they’re symmetrical. But why? How does this work? I offer a brief tentative hypothesis. This article is public domain. Feel free to copy all of the text without attribution (although some pictures may have attribution requirements.) — July 15, 2011 — tom sulcer

Actress Ming Na. Why is she beautiful?
Note: This thinking came out of my contributions to Wikipedia when I was helping with articles about  Dating and Physical Attractiveness as well as my own Google “knol” article called Dating and mating in the twenty tens. Scientists may know this but then why haven’t I come across it yet in my explorations? It’s possible that my hypothesis here is (1) wrong or (2) right and (2a) new knowledge or (2b) old knowledge. Which is it? I am not sure. Perhaps I haven’t yet read enough? — tom sulcer July 15, 2011

I am a handyman. I fix stuff. And it’s highly important for handymen to study subjects like beauty so we can make houses look better. When houses are symmetrical, they’re prettier; same thing with human faces. But why does symmetry cause us to see things as more beautiful?

I propose a tentative hypothesis with four interlocking components:

First, when a face is symmetrical — so that the left and right sides mirror each other along a vertical axis — then the mirroring of left and right makes it very easy cognitively for a human mind to tell IF the two sides match since there is a visual copy of each side right there to see. Does the left match the right? It’s obvious. It only takes a split second for the brain to process this information visually and make a correct determination about whether the face is, in fact, symmetrical. Aberrations, distortions, lopsided features — these show up instantly as well, again, because there is a readily viewable copy to judge — the opposite side — in plain view.

Suppose beauty was not based on symmetry. Or suppose human faces were not symmetrical. Then how would we know if a face was beautiful? Maybe we could compare the face we’re looking at in the present against some unseen but remembered beauty standard from the past. [1] But this is difficult conceptually.[2] We’d have to remember the standard. It’s much much easier conceptually to see whether left matches right.

Second, split-second beauty assessment is highly important for reproduction, particularly for males, since a male who can decide which woman is beautiful in the shortest amount of time has a reproductive advantage over other males. He can woo her first. He can focus his attention productively on the fittest females. The point here is that symmetry enables split-second beauty assessments since it requires less mental processing. And beauty correlates positively with many other attributes such as intelligence,[3] [4] disease-resistance,[5] health,[6] income, [7] [8] happiness,[8] and potential reproductive ability,[9] and is a sign of a healthy injury-free and disease-free upbringing.[5]

Third, these ideas fit with others which I think are better known. Specifically, as a human grows from a zygote to a fetus to a baby to a toddler to a teenager to an adult, that it is a difficult task genetically for the developing body to keep both sides symmetrical. Billions of cells are reproducing. Left and right sides must match exactly. The timing must be right. If the left rushes ahead even slightly faster than the right side, distortion can result. The stages of development must be followed in precisely the right order, so that inner structures are built first, followed by intermediate structures and then outer ones such as the skin. This is a difficult task for the DNA. Any weird biological experiences — a gash, a wound, a disease — can throw off this sequence on one side possibly and lead to an aberration or lopsided feature. Malnutrition can distort one side or process too. Any genetic mutation which affects one side but not the other can distort the symmetry. When symmetry is successfully achieved despite these numerous environmental and developmental challenges, it is a visible signal of genetic health — that the DNA effectively got the billions of steps right, in the right order, without trauma, without disease.

Fourth, computer scientist and software entrepreneur Gary Robinson suggests an additional genetic benefit of symmetry: he wrote “it takes far less information storage to produce an inverse copy of something that to produce two completely different things.”[10] It takes less DNA to come up with more human. So persons with symmetrical features can be described with shorter strands of the genetic helix. It can be argued that shorter DNA strands are a competitive evolutionary advantage. Shorter DNA strands means …

  1. it takes less time for the right proteins within a cell to find each other since they’re closer together. Shorter distances enable faster processing. Cells can make RNA copies and build other needed proteins faster as a result.
  2. there is less room for mistakes in the sense that extra clutter (junk DNA?) enables more chances for harmful mutations (e.g. if an inactive gene area is switched “on” by mistake by an operator gene). Organisms are less likely to be afflicted with a run-away genetic mistake such as cancer.
  3. there are fewer proteins required when full-length copies are made — meaning an organism can effectively eat less.

DNA is like a computer program. Both are sequences of instructions.  And a tight computer program uses less computer storage, permits a smaller microprocessor, uses less electricity, works faster, easier to debug, less errors. The same might be true for DNA — tighter “code” is a helpful evolutionary advantage. Isn’t it more efficient? And wouldn’t that be a competitive advantage? I think it must be.

However, a biologist disputes this fourth hypothesis, saying that asymmetrical patterns can be derived with the same efficiency as a symmetrical one. Here is the argument:

It doesn’t take twice the DNA to specify an asymmetrical organism, because DNA isn’t like a house blueprint. Think of it like a recipe, except DNA contains the ingredients, instructions, and tools, all at once. Some genes are just structural, forming the actual “stuff” in us, but others are the “tools” and “instructions” which specify what happens when and guides the process. The entire process of early embryonic development, which lays down the majority of your body plan, is governed by a small handful of “tool” genes that govern simple steps like defining anterior/posterior, top/bottom, segmentation, limb buds, etc. Dramatic shifts in morphology can be accomplished simply by turning these genes on/off in new ways, with no underlying change in the DNA of either the tool genes or the simple structural genes.
Another way of looking at it is to consider fiddler crabs – they don’t, so far as I know, have unusually big genomes, nor is there a difference between male and female genomes (obviously). Yet males exhibit tremendous asymmetry due to their giant claw, while females are typical bilateral organisms. The reason is that you don’t need a whole new set of genes for left and right, you just need a left-vs-right signal, a limb ID signal, a sex signal, and a gene that, if present in the left (or right) chela (pincer claw) of a male causes massive hypertrophy.
To further illustrate it, consider a zebra’s stripes. Different zebra species have different stripe patterns, some with broad stripes others with narrow. But the position of every stripe is not coded into the DNA. Instead, you have a simple series of genes that say “on day X of embryonic development, make alternating stripes 20 cells wide”. All of the subsequent variation in stripe shape is due to subsequent warping and growth of the embryo, and differences between individuals due to slight, normal variations in the number of cells present at a given stage. Narrow vs. wide stripes can be explained as simply turning the gene off earlier or later – if you form 20 cell wide stripes on an embryo 200 cells long vs one 400 cells long, the latter will have more stripes, and that’s preserved as the organism grows up. So an incredibly complex pattern can arise from just a few genes.  — User “Mokele” (Wikipedia handle)  [11]

So I am less sure about the fourth hypothesis.

Note: facial symmetry, while important,[12] [13] [14] is not the only characteristic determining physical attractiveness, of course, but it is one of many visual cues such as symmetry of the entire body, height, body weight, hair color, voice, scent, relative position of facial features, proportionality of facial features,[14] hormonal cues regarding ovulation,[15] and so forth. That is, symmetry, by itself, is not enough to determine beauty, but is one aspect of many which helps determine beauty. It is possible to have a highly symmetrical yet unappealing face, if other aspects (such as nose or eyebrow position relative to other facial features) vary too much from a standard sense of what a face is supposed to look like. While the strong consensus position is that symmetry is highly related to beauty, one opposing view is that excessive or extreme symmetry, or “absolute flawlessness” is “disturbing” since it offers “no point of connection.”[16]

Overall, in effect, symmetry suggests genetic health. [17] [18]  A symmetrical face is perceived as more beautiful and helps create desire[19] in the minds of possible sexual partners. It is a clear indication to potential mates that the DNA is functioning properly and that mating with such a person will likely result in similarly healthy offspring. Humans who appear symmetrical and beautiful, and humans who have the perceptual brainpower to correctly determine whether another human has symmetrical and beautiful features, have competitive advantages in the reproductive rodeo. They get a better chance to pass forward quality genes to future generations.

Let me illustrate graphically. I found three faces of beautiful women from pictures from Wikimedia Commons. [20] [21] [22]  In some cases I shifted the picture to make the face vertical. Then I drew a line down the middle. So it’s easy to see if left matches right. With these internationally-renowned beautiful women, it’s easy to tell this. But the symmetry is not totally perfect — even with these beautiful women, the mirroring isn’t exact.

Real face.
Distorted image.

Here’s Angelina Jolie. The line suggests mirroring is nearly exact on the right image. Next, I used picture editing software on the real face photo to distort the image. In the distortion, it is readily apparent that something is unbalanced, off, asymmetrical.

Real face.

Here’s Indian actress Genelia D’Souza. Same approach, but this time the left side around the mouth was distorted by the photo editing software. When movable parts of the face are distorted, such as the mouth or eyebrows, then it’s harder to detect whether it’s a deformation or whether someone is smiling or raising an eyebrow. But when a relatively immobile feature such as the eyes or nose is distorted, a malformation is readily apparent.

Real face.

Actress Ming Na. Same thing. Here the distortion is more pronounced, possibly because we pay greater attention to the shape of the eyes, and because the eyes do not move like the mouth or eyebrows.

Real Taj Mahal. [23]  Built right.
Distorted. Did somebody leave
the water running on the second

And the idea that symmetrical things are more beautiful extends to other areas as well, including buildings. When we stand in front of a symmetrically-built structure, it is easier for us to judge whether construction was done properly. Left and right match.

As scientists more fully understand the brain, and get a better handle on cognitive processing, perhaps we will understand how these processes happen, but my best guess at this point is that the left-and-right mirroring of symmetrical features enables fast and accurate cognitive processing, and fits in with a model of split-second beauty assessments being a competitive reproductive advantage. — tom sulcer july 16, 2011

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