Facial symmetry is the degree to which the features on each side of the human face line up and match in both size and orientation. The symmetry of one's face has a number of implications, including aesthetic and overall healthiness. It is part of the larger field of human body symmetry, which is believed to play a role in how people determine attractiveness.
In a basic sense, facial symmetry is literally how closely the size, angle, and location of one's eyes, ears, nostrils, cheeks and other duplicated features mirror each other on the left and right sides. It is very rare for both eyes, for example, to be exactly the same size or distance above the nose. Most people do not consciously notice common, minor asymmetry.
Scientists believe that facial symmetry is one of a number of physiological traits that subtly influence attractiveness. Others include perceived youthfulness and similarity to the norm, known as averageness. Each of these elements indicate on a biological scale an individual's suitability for reproducing. Uniquely among humans, attractiveness is not limited to these factors, and exceptions abound.
Although facial symmetry is governed to a large degree by heredity and genetics, there are other factors that can affect the orientation of one's features. Diseases such as Bell's palsy might cause muscular atrophy in the face, leading to asymmetry. In addition, birth defects or trauma such as mandibular condylar hypoplasia can affect the orientation of facial features.
Various types of cancer also might affect facial symmetry, sometimes drastically. Oral cancer, in particular, routinely requires amputation of part or all of the jaw bone. This procedure has a profound effect on the shape of the face and how its features line up. Prosthetic jaws are commonly prescribed to help return a large degree of symmetry to the face.
Changes in facial symmetry also typically occur following a stroke. Unlike a genetic defect in which the face develops asymmetrically from birth, a stroke victim might suffer nerve or muscle damage that causes one half of the face to be paralyzed. As a result, its features droop and become asymmetrical.
Facial symmetry, whether compromised naturally or through trauma such as a car accident, can be improved to a degree through plastic surgery. Science cannot greatly alter the intrinsic bone structure of the human skull, but there are a variety of procedures, from the mild to the extreme, that can actually improve symmetry or at least give the impression of greater symmetry. Even a stroke victim, through physical therapy, can regain muscular control of the face, leading to renewed facial symmetry.