Key Terms in Evolutionary Biology of Sex, Gender, and Sexuality: Symmetry

= Definition =

Symmetry is a fundamental concept that pervades science and culture. It has two parallel definitions: the first definition pertains to the material, and the second definition pertains to the immaterial. When a physical object is symmetric, two or more of its parts are indistinguishable from each other (whether or not exactly identical), and this can be determined by precise systems of measurement, such as geometry. Symmetry can be measured in multiple directions, such as radially or bilaterally. For example, when judging the symmetry of a human face, as evolutionary biologists do, the face is determined to be symmetric if it is bilaterally symmetric -- that is, if the right and left sides of the face match up. But symmetry does not merely describe the proportions of material substances, but also less explicitly definable occurrences of harmony or balance, such as in music and reciprocal social relations.

Humans prefer both material and immaterial symmetry to asymmetry, or the lack of symmetry. Symmetry often elicits a positive response in humans, inspiring a sense of harmony, balance, and unity, and is thus often seen as a manifestation of ideal beauty, perfection, and even the divine. For example, Aristotle attributed a spherical shape to the heavenly bodies, linking the attribute of a formal measure of symmetry to the natural order and divine perfection of the cosmos.

= History =

The concept of symmetry has a long history in Western philosophy. In Plato’s (428 – 328 BC) Symposium, Diotima argues that one must begin their spiritual and intellectual journey by first learning to appreciate and understand physical beauty. With this basic understanding of beauty, Diotima argues, humans will then be able to take notice of divine beauty. This concurs with Plato’s own theory of Forms, which states that physical objects (“forms”) are manifestations of the immaterial, immutable, abstract ideas (“Forms”) that are the true basis of reality; the material is a mere “shadow” of the immaterial. Aristotle (384 – 322 BC), a student of Plato, builds on this view in Metaphysics (c. 350 BC), stating that “the chief forms of beauty are order and symmetry and definiteness, which the mathematical sciences demonstrate in a special degree.” The beauty of the human face in particular captivated Aristotle, who believed that a careful study of it could reveal one’s underlying mental character and health. He is thus regarded as the founder of physiognomy, the study of a person’s outward appearance to determine personality characteristics.

The idea that symmetry is an indicator of beauty has a very long legacy, and it remains prominent to this day. Various arguments by many different philosophers (Ruskin, Hogart, Hegel, Dewey, Félibien, Abrabanel, Addison, et. al) have since reinforced the idea that symmetry is essential to beauty (and others have countered it). The Greek philosopher and mathematician Pythagoras’s discovery that the human body conforms to the golden ratio (link) had tremendous influence on Greek art, which sought to abide by these laws of proportion and symmetry. Although the ideas of a geometric basis for beauty existed in the classical era, it flourished during the Renaissance. In the 16th century, the Italian geometer Luca Pacioli rediscovered the “golden secret” and wrote an influential book on it – De Divina Proportione (1509), illustrated by Leonardo da Vinci, who himself had previously displayed an ardent interest in the mathematics of art and nature. Da Vinci’s famous Vitruvian Man (1487) is a key example of the influence of classical scholars like Vitrivus, on Renaissance art. Other seminal Renaissance artists (e.g., Michelangelo, Raphael, Durer) were highly influenced by this idea, and accordingly sought to symmetrize their artwork.

= Scientific background =

Efforts to associate the outward appearance with the underlying “character” of things, especially people, have existed throughout history. The first published account of an effort to create a systematic method to do so was Physiognomica by Aristotle (384 – 322 BC). He discusses observations of human physical appearance that correlate with their behavior, and from these observations, universal standards are deduced. In this text, Aristotle first proposed the science that became physiognomy, which is defined as the art of determining character or personal characteristics from the form or features of the body, esp. of the face.

In the nineteenth century, physiognomy became immensely popular and underwent its heyday, accepted as a legitimate science by most for the first time. But by the time the twentieth century came, it had fallen out of favor again, and was dubbed as a pseudoscience.

A recent article in the New Scientist reported that "“[physiognomy] is undergoing something of a revival. Researchers around the world are re-evaluating what we see in a face, investigating whether it can give us a glimpse of someone's personality or even help to shape their destiny. What is emerging is a ‘new physiognomy’ which is more subtle but no less fascinating than its old incarnation.”" Many critics have recently voice dissenting opinions on the "physiognomy revival" that seems to be taking place within HBE circles.

HBE Theorists
The past two decades have seen a proliferation of scientific literature on the subject of symmetry’s relevance to physical attractiveness and mate selection. This is based on the “good genes hypothesis”, which states that our mating preferences have evolved to help us select mates of superior genetic quality. That is to say, a person’s exterior is an indication of the interior, or more bluntly, that pretty people are healthier. The hypothesis seeks to explain the adaptive value of our preference for certain physical traits (including symmetry, Waist-Hip-Ratio, youthfulness, hormonal markers), and rests on the assumption that these preferences are inherent to human nature, and that to a certain extent, a universal, cross-cultural and cross-temporal definition of beauty exists.

The hypothesis states that there is adaptive value to a preference for symmetric bilateral traits in our mating partners because such traits can indicate an overall high quality of development. Symmetry can be seen as an indicator of overall health and the ability to resist environmental antagonists like mutations, pathogens, and toxins, which can all hinder the natural development of symmetric features. Evidence to support this theory include: "- Research that shows that symmetric features are partly heritable, and is thus caused by specific genes. - In various species, males who are more symmetric have more sexual partners than asymmetric males. Furthermore, symmetric males are more likely to be sexually satisfying for their partner and give them more orgasms. - When women are ovulating (the time when conception is most likely), they prefer the scent of symmetric men to less symmetric men. - Various studies that show a positive correlation between physical attractiveness (determined by participants’ ratings of photos) and calculated symmetricity."

= Debates =

The theory that symmetry is an indicator of good genes is highly controversial within academic circles. Scientists have offered a multitude of possible explanations

Do we actually prefer symmetry? Most scientists agree that humans tend to prefer the symmetrical to the asymmetrical. But one study by Swaddle &amp; Cuthill (1995) provides radically subversive evidence: facial asymmetry, and not symmetry, was positively correlated with rated facial attractiveness. The authors admit that this was not the result they were expecting, and that they remain inconclusive about symmetry’s influence on judgments of physical attractiveness.

Even if the finding of this study is disregarded, doubt remains. A common point of critique is the methodology of the studies in question. Points of concern include:

"- One study included pictures of people with drastic differences between “symmetric” and “asymmetric” (extreme, rare examples). - Only looking at each possible indicator of physical attractiveness (symmetry, averageness, hormone markers) individually, when they most likely interact with each other in a very complex manner. Thus, to attempt to extract the importance of a single feature is a much more complicated procedure, if not impossible. - Need studies that have more accurate sampling (need more cross cultural, temporal evidence), and that acknowledge completely contrary, drastically different beauty ideals from different culture." If we are looking for symmetry, we will find it, especially if we have already decided that it is favorable to asymmetry. Given the deliberate search for symmetry throughout history, it may be a self-fulfilling prophecy. In the quest to find symmetry, we may oversee asymmetrical counterevidence. This methodological danger is likely, but the degree to which it affects relevant data is unknown.

If we do prefer symmetry, why? But overall, it is accepted that humans do indeed prefer symmetry to asymmetry when choosing mating partners. There is without a doubt something that causes people to associate symmetry as attractive, but how this translates to mate quality is highly debatable. The precise explanation for why we prefer symmetry, rather than the question of if we prefer it, is the more common topic of debate, since most scientists accept that we do indeed prefer symmetric faces. Hypotheses regarding why we prefer symmetric faces include: "- Good genes hypothesis: our mating preferences have evolved to help us select mates of superior genetic quality. - Perceptual bias: symmetry is easier to process visually, and thus the mechanics of our visual system favor symmetry. - Covariant hypothesis: symmetry may simply covary with attractive (and evolutionarily advantageous) features, rather than act as a primary cue. - Maybe humans are just attracted to symmetry in general, and we are not attracted to it because it is sexually advantageous." Overall, perhaps too much emphasis is placed on symmetry’s role in our judgment of physical attractiveness. It is indubitable that there are many factors that contribute to our notions of beauty, and symmetry is [likely] to be one of them, but only one of them. Indeed, an ugly face can be symmetric and a beautiful face can be assymmetrical, so the degree to which symmetry plays a part in our judgment of physical attractiveness is uncertain.

Disregarding other factors, or the covariance of these factors (rather than studying them independent of each other), is dangerous. More evidence is needed for many of these claims, and the question of whether beauty is merely skin-deep or if it is in fact “in the adaptation of the beholder.”

= Notes =

= References =

Aristotle, Metaphysics.

Barash, D. P., &amp; Lipton, J. E. (2009). How women got their curves and other just-so stories: Evolutionary enigmas. New York: Columbia University Press.

Cárdenas, R. A., &amp; Harris, L. J. (2007). Do women's preferences for symmetry change across the menstrual cycle? EVOLUTION AND HUMAN BEHAVIOR. 28 (2), 96-105.

Cleese, John (Writer), Erskine, J. (Director), Stewart, D. (Director). (2001). Beauty (Television series episode). In George, S. (Producer), The Human Face. Burlington, NC: British Broadcasting Corporation. Retrieved from YouTube. 06 Nov. 2009. 24 Mar. 2010 &lt;http://www.youtube.com/watch?v=tKv-F3oUkz0&gt;.

Cowley, G. (1996). The Cover: The Biology of Beauty. Newsweek. 127 (23), 60. Etcoff, N. L. (1999). Survival of the prettiest: The science of beauty. New York: Doubleday. Fink, B., Neave, N., Manning, J. T., &amp; Grammer, K. (2006). Facial symmetry and judgments of attractiveness, health and personality. PERSONALITY AND INDIVIDUAL DIFFERENCES, 41 (3), 491-499.

Fink, B., &amp; Penton-Voak, I (2002). Evolutionary Psychology of Facial Attractiveness. CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE. 11 (5), 154-158.

Gangestad, S.W., Thornhill, R. &amp; Yeo, R.A. (1994). Facial attractiveness, developmental stability, and fluctuating asymmetry. ETHOLOGICAL SOCIOBIOLOGY. 15, 73-85.

Grammer, K., Fink, B., Mller, A. P., &amp; Thornhill, R. (2003). Darwinian aesthetics: sexual selection and the biology of beauty. BIOLOGICAL REVIEWS, 78(3), 385-407.

Grammer, K., &amp; Thornhill, R. (1994). Human (homo sapiens) facial attractiveness and sexual selection: The role of symmetry and averageness. JOURNAL OF COMPARATIVE PSYCHOLOGY, 108, 233-242.

Halberstadt, J. &amp; Rhodes, G. (2000). The Attractiveness of Nonface Averages: Implications for an Evolutionary Explanation of the Attractiveness of Average Faces. PSYCHOLOGICAL SCIENCE. 11 (4), 285-289.

Little, A. C. &amp; Jones, B. C. (2003) Evidence against perceptual bias views for symmetry preferences in human faces. PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON B, 270, 1759-1763. Manning, J. T., Scutt, D., Whitehouse, G. H., &amp; Leinster, S. J. (1997). Breast Asymmetry and Phenotypic Quality in Women. EVOLUTION AND HUMAN BEHAVIOR. 18 (4), 223-236.

Moller, A.P. &amp; Thornhill, R. (1997). Developmental stability and evolution. Oxford University Press.

Osborne, H. (1986). Symmetry as an aesthetic factor. Computers &amp; Mathematics with Applications, 12(1-2), 77-82.

Penton-Voak, I. S., &amp; Perrett, D. I. (2000). Female preference for male faces changes cyclically. EVOLUTION AND HUMAN BEHAVIOR. 21 (1), 39-48.

Plato, Symposium.

Rhodes, G., Proffitt, F., Grady, J. M., &amp; Sumich, A. (1998). Facial symmetry and the perception of beauty. PSYCHONOMIC BULLETIN AND REVIEW, 5, 659-669.

Scheib, J. E., Gangestad, S. W., &amp; Thornhill, R. (1999). Facial Attractiveness, Symmetry and Cues of Good Genes. PROCEEDINGS: BIOLOGICAL SCIENCES, 266(1431), 1913-1917.

Synnot, A. (1989). Truth and Goodness, Mirrors and Masks – Part I: A Sociology of Beauty and the Face.THE BRITISH JOURNAL OF SOCIOLOGY. 40 (4), 607-636.

Swaddle, J., &amp; Cuthill, I. (1995). Asymmetry and Human Facial Attractiveness: Symmetry May not Always be Beautiful. PROCEEDINGS: BIOLOGICAL SCIENCES. 261, 111-116.

Thornhill, R., Gangestad, S.W. &amp; Gomer, R. (1995). Human female orgasm and mate fluctuating asymmetry. ANIMAL BEHAVIOR. 50, 1601-1615.

= Author =

B. Talbot