Sexy Symmetry

My recent post on attraction was, by a large margin, my most popular post yet. As this is an area of expertise for me, I’m happy to write more on the topic. This post will discuss in greater detail an aspect of attractiveness that I touched on briefly before: symmetry.

Leonardo da Vinci’s “Vitruvian Man” exhibits high symmetry. Image from http://blsciblogs.baruch.cuny.edu

The human body is designed to have external symmetry. There are certain parts of us that are designed to be asymmetric — e.g. the brain, most of our organs — but for the sake of this conversation, “symmetry” refers to the parts of us that are supposed to be symmetrical. Evolution built our bodies to be perfectly symmetrical on the outside, and there is good reason for this. For example, if our legs were not the same length or equally-muscled, walking around would be inefficient. Overall, our symmetry allows us to have mechanical balance in our bodies, and to interact with our surroundings with both sides of our body (handedness not withstanding).

That we end up basically symmetrical as adults is remarkable. When our bodies grow, they do so by the individual cells in our body dividing, growing, differentiating, and stacking up to form our limbs and organs. This process takes place over one and a half decades, and involves roughly 10 trillion cells in the end product (not counting the trillions that die along the way). A symmetrical body therefore has roughly the same number of cells in the same locations on each half of the body. This is like two cars with blind-folded drivers traveling perfectly parallel and at the same speed for hundreds of miles. But our bodies do not turn out with perfect symmetry. Developmental perturbations, such as injury or disease, can disrupt the intended course of growth. If one of the drivers in my example runs over a small rock, his course and speed will be altered slightly, and he will deviate from the other driver. The number and intensity of these perturbations will determine how different the direction and location of the two drivers will be at any given point.

Thus body symmetry is an indicator of what we biologists call “phenotypic quality.” This is just a fancy way of saying how well-built a body is. Having higher symmetry means that you were able to prevent your body from being disrupted by developmental perturbations. Indeed, people with higher symmetry have higher intelligence, faster reaction time, are more masculine (if male) or more feminine (if female), are healthier, have better mental health, better mental acuity later in life, have higher sperm quality (only in men, obviously), and have better athletic ability. This is not a comprehensive list. The relationship between symmetry and these traits is sometimes small, but there is a clear relationship overall. Symmetry does not cause these traits, but symmetry is related to these traits because they all result from a body that is well-built.

And this is why more symmetrical people are more attractive.

Success in natural selection is about leaving more surviving offspring than other members of your species. If your offspring are put together better than the offspring of other individuals, they will survive better and ultimately leave more surviving offspring themselves. Choosing mates with high phenotypic quality will increase the chance that the chooser’s offspring will have high phenotypic quality. Resistance to disease is an aspect of phenotypic quality that is of particular importance in a mate. Remember from my post on sexual reproduction that an interest in producing disease-resistant offspring is a primary reason why sexual reproduction evolved in the first place.

For this reason, humans are not alone in our preference for symmetrical mates. This preference has been found in many species of birds, fish, insects, spiders, and even plants. The preference for symmetrical mates probably evolved shortly after sexual reproduction did. In my previous post, I list attraction to symmetry as a species-typic trait, but it is typical of way more species than just humans. An interest in high quality mates is integral to sexual reproduction.

 

The female peahen prefers peacocks that have more symmetrical “eyes” in their tails. This one looks pretty good. Image from http://gallery.photo.net

Symmetry is important, but it is only one trait of many that are involved in attractiveness in humans. As I write more about attraction, I hope the ways in which these traits interact will become more clear.

 

Have a topic that you want me to cover? Let me know in the comments section.

Follow me on twitter @CGEppig

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About Christopher Eppig, Ph.D.

I have a Ph.D. in biology and a passion for sharing my knowledge and understanding of the natural world with anyone who will listen. At a time where science is permeating public life more than ever, it is especially important that the public understand what science is, and how its findings intersect with their own lives. In addition to the more practical benefits of scientific literacy, I believe strongly that understanding the natural world enriches peoples lives. The man behind the curtain is not me — it is the real world, which we can discover through science, and it is beautiful. Let me show it to you.  Follow me on twitter @CGEppig. View all posts by Christopher Eppig, Ph.D.

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