Symmetry Part 1: Introduction
“Beauty is our weapon against nature; by it we make objects, giving them limit, symmetry, proportion. Beauty halts and freezes the melting flux of nature.”
In the study of attractiveness, symmetry almost always refers to bilateral, mirror, or reflection symmetry: each side is identical to the other when split down the middle – as is the figure on the left. The midsagittal plane – the plane that runs through the body, beginning at the head and passing through the spinal column and the navel, is the marker of mirror symmetry. Bilateral symmetry is common in vertebrates and chordates. It promotes movement, a central nervous system, and cephalization (the development of heads).
While symmetry is expected in development, asymmetry (the absence of symmetry) is common. There are three types of asymmetry: directional asymmetry, antisymmetry, and fluctuating asymmetry (Kowner, 2001).
Directional Asymmetry (DA): some traits develop more on one side than the other, e.g., the human brain.
- Antisymmetry: asymmetric development is typical, but unpredictable, e.g., claw size in fiddler crabs.
- Fluctuating Asymmetry (FA): “randomly produced deviations from perfect symmetry of two sides of quantitative traits in an individual for which the population mean of R-L differences is zero and their variability is near-normally distributed” (Kowner, 2001, p.448).
By-and-large, DA and antisymmetry are understood to contribute to an organism’s fitness. FA, on-the-other-hand, is generally assumed to be an indicator of developmental instability: the inability “of the organism to resist or buffer the disruption of precise development by environmental and genetic stresses” (Kowner, 2001, p. 447). That is, perfect symmetry is the developmental expectation and organisms with genetic weaknesses in particular environments will show this weakness through FA (irregular development).
Clear connections between facial and body symmetry and attractiveness exist. Connecting the dots:
- we like symmetry
- symmetry indicates developmental stability (the inverse of instability)
- developmental stability means good genes
Thus, the preference for symmetry is a preference for good genes. This conclusion is straightforward and coherent – but may be premature:
- the connection between FA and DI is “poorly substantiated” and
- models connecting FA and DI are currently tentative and exploratory.
To be clear: I am not opposed to the possibility that there is a connection between FA and DI. I am merely suggesting that beauty researchers emphasize the tentative nature of this connection.
Planar image courtesy of Yassine Mrabet
Kowner, R. (2001). Psychological perspective on human developmental stability and fluctuating asymmetry: Sources, applications and implications British Journal of Psychology, 92 (3), 447-469 DOI: 10.1348/000712601162284