Symmetry (physical attractiveness)
In evolutionary psychology, symmetry especially facial symmetry is one of a number of aesthetic traits, including averageness and youthfulness, associated with health, physical attractiveness and beauty of a person or non-human animal.  It is also hypothesized as a factor in both interpersonal attraction and interpersonal chemistry. Animal studies show that diseased mothers give birth to offspring that show greater asymmetries.  From human studies, it is known that women with asymmetrical breasts are less fertile than those with greater symmetry.

# Health and physical attractiveness
Edler  cited research supporting the claim that bilateral symmetry is an important indicator of freedom from disease, and worthiness for mating.  Facial asymmetries and minor physical anomalies begin to appear early in embryonic development, mainly the first trimester of pregnancy, and can be a sign of instability during this growth.  Fluctuating asymmetry (random differences between two sides, as opposed to the deliberate natural asymmetry in some animals) develop throughout the lifespan of the individual and is a sign of the phenotype being subjected to some levels of stress.
The ability to cope with these pressures is partly reflected in the levels of symmetry. A higher degree of symmetry indicates a better coping system for environmental factors.  While the visible signs of this may not be particularly apparent, it is thought that they have at least a subconscious effect on people's perception of their beauty.  Zaidel et al. in an empirical study upholds the claim that facial symmetry may be critical for the appearance of health.  Their study disputes, however, the beauty or attractiveness claim.
Facial symmetry is neither the only trait nor is it necessarily the most important trait of what a culture considers attractive.  The competing aesthetic theory of wabi sabi posits the existence of beauty through imperfection.

# Interpersonal attraction
Suzi Malin, a British portrait painter, wrote Love at First Sight (ISBN 0-7566-0401-X) to examine the relationships of 50 celebrity couples.  She speculates that there are three aspects of facial symmetry which attract one person to another: echoism, harmonism and prima copulism.  Echoism is the similarity of one or more facial features, particularly the eyes, eyebrows, nose and mouth.  Harmonism is a similarity in facial proportions, usually involving the features and the distance between them.  Prima copulism is an attraction to another person based on them having similar features to someone with whom they formed a strong interpersonal bond with at an early age, usually a mother or father.
In "Symmetry and Human Facial Attractivess" (University of St Andrews 1999) by David I. Perrett, D. Michael Burta, Ian S. Penton-Voaka, Kieran J. Leea, Duncan A. Rowlanda and Rachel Edwards it is posited again that symmetry may act as a marker of phenotypic and genetic quality and is preferred during mate selection in a variety of species. Measures of human body symmetry correlate with attractiveness, but studies manipulating human face images report a preference for asymmetry. These results may reflect unnatural feature shapes and changes in skin textures introduced by image processing. When the shape of facial features is varied (with skin textures held constant), increasing symmetry of face shape increases ratings of attractiveness for both male and female faces. These findings imply facial symmetry may have a positive impact on mate selection in humans. This, and other recent scientific findings, have been tested empirically on perceived "beautiful people" in Hollywood and researchers found that on a percentage basis the highest rating of symmetry was achieved by actress Cate Blanchett.

# Historical perspective
This normative study of the bone structure of the face and skull places it in the intellectual traditions of Phrenology and Craniometry.  These earlier traditions ambitiously sought to assess character, personality, or intelligence from a person's bone structure.  By contrast, the current study primarily focuses on aesthetic concerns that, if misapplied, would not result in infringement of a person's legal or civil rights.
Since the 1990s, descriptive studies of human facial bone structure underlie the practice of face recognition systems using software for security applications.  The reliability of these systems has been brought into question.