Facial bilateral symmetry is typically defined as fluctuating asymmetry of the face comparing random differences in facial features of the two sides of the face. The human face also has systematic, directional asymmetry: on average, the face (mouth, nose and eyes) sits systematically to the left with respect to the axis through the ears, the so-called aurofacial asymmetry.
Directional asymmetry is a systematic asymmetry of some parts of the face across the population. A theory of directional asymmetries in the human body is the axial twist hypothesis. As predicted by this theory, the eyes, nose and mouth are, on average, located slightly to the left of the axias through the ears. This aurofacial asymmetry is very small in young adults (0.5 degree), but much larger in small children (4 degrees).
While studies employing the composite faces produced results that indicate that more symmetrical faces are perceived as more attractive, studies applying the face-half mirroring technique have indicated that humans prefer slight asymmetry. Also, studies have shown that nearly symmetrical faces are considered highly attractive as compared to asymmetrical ones. The symmetry of the nose seems to be more important than that of the lips
Highly conspicuous directional asymmetries can be temporary ones. For example, during speech, most people (76%) tend to express greater amplitude of movement on the right side of their mouth. This is most likely caused by the uneven strengths of contralateral neural connections between the left hemisphere of the brain (linguistic localization) and the right side of the face. Also, lying distorts the symmetric balance of the facial musculature.