Facial symmetry

Facial symmetry, is one element of bodily symmetry, including fluctuating asymmetry. Along with traits such as averageness and youthfulness it influences judgements of aesthetic traits of physical attractiveness and beauty, and is associated with fitness-linked traits including health, It is also hypothesized as a factor in both interpersonal attraction and interpersonal chemistry.

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. Random differences between the two sides, known in biological terms as Fluctuating asymmetry, and not deliberate asymmetrical structures found in some animals, develops throughout the lifespan of the individual and is a sign of the phenotype being subjected to some levels of stress.

Biologists, based on work with animals mostly, are exploring new links between facial symmetry and physical attractiveness. When a face is symmetrical vertically, so that left and right sides mirror each other along a vertical axis, the mirroring makes it easy cognitively and perceptively for a human mind to tell if the two sides match. There is a visual copy of each side in plain view making it easy for a person to judge if left matches right. The mind can make this mental comparison in a split second to look for aberrations, distortions, or lopsided features. If faces were not symmetrical, then the mind would have a more challenging cognitive task of comparing the seen face with a remembered standard face. The split-second analysis of symmetry is a reproductive advantage, particularly for males, since a male can decide quickly who are the most beautiful females and court them first and spend more time with them. An additional benefit to symmetry is because it takes less information storage since one strand does double duty; in effect, less DNA is needed to make more of a human being and brings the benefit of shorter DNA strands and biological efficiency. Overall, symmetry is a rather difficult challenge to achieve biologically since the body has to coordinate a perfect match for each side when literally billions of cells are reproducing over years. Cell growth must happen in exactly the right order and speed so that left and right sides match exactly. If the left side rushes ahead even slightly faster than the right side, distortion can result. Inner structures must be built first, followed by intermediate and later tertiary structures such as the skin. Any weird biological experiences -- a wound, disease, malnutrition, a mutation -- can throw off this sequence on one side. When symmetry is achieved successfully despite numerous challenges, it is a visible signal of genetic health and is attractive physically.

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 an unconscious 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.

Interpersonal attraction
A large body of work has been conducted relating symmetry to attractiveness, though this is not yet incorporated in this article. Symmetry may act as a marker of phenotypic and genetic quality and is preferred during mate selection in a variety of species.

Experimentally, 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.

Non-scientific theories of attraction and symmetry abound. For instance the conjunction "of similar and related parts that are to some extent contrasted to one another" has been considered to have the most aesthetic appeal. Likewise shared features echoism, proportions and similarity to people the person bonded to early in life (prima copulism) have been suggested as relevant.

Congenital defect

 * Facial hemi-trophy or hypertrophy of superfacial tissue, muscle & bone.
 * Mandibular Condylar hypoplasia due to intrauterine or birth trauma.
 * TMJ Ankylosis, the mandible moves to the affected side.
 * Mandibular body or ramus hyperplasia, the mandible moves to the unaffected side.

Traumatic
Zygomatic process fracture & followed infra orbital depression.

Inflammatory
Abscess - cellulitis - cyst

Muscular
a- Atrophy of facial musculature following Bell's palsy.

b- Hyperplasia of masseter muscle in clenching habit.

c- Patients using only one side in chewing

Salivary Glands
Inflammatory as mumps or neoplastic.

Neoplastic
Ameloblatoma - lipoma - osteoma