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One specific measure of bodily symmetry
the physiognomical concept

Facial symmetry is one specific measure of bodily symmetry. Along with traits such as averageness and youthfulness, it influences judgments of aesthetic traits of physical attractiveness and beauty. For instance, in mate selection, people have been shown to have a preference for symmetry. 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 systematic. The average across the population is not "symmetric", but statistically significantly biased on one direction. That means, that individuals of a species can be symmetric, or even asymmetric to the opposite side (see, e. g., handedness), but most individuals are asymmetric to the same side. The relation between directional and fluctuating asymmetry is comparable to the concepts of accuracy and precision in empirical measurements. There are examples from the brain (Yakovlevian torque and spine, and inner organs (see axial twist theory), but also from various animals (see Symmetry in biology).

On average, the aurofacial asymmetry is slightly larger for the eyes than for the nose, as shown by the figure. [[File:Age relation for aurofacial asymmetry. png|thumb|upright=1.5|The gradual decrease of the aurofacial asymmetry with age until the beginning of adulthood. Source: figure 3 of reference and face half mirroring have been among the techniques used. Conclusions derived from face mirroring, however, have been called into question, because it has been shown that mirroring face halves creates artificial features. For example, if the nose of an individual is slightly bent to the right side, then mirroring the right side of the face will lead to an over sized nose, while mirroring the left side will lead to an unnaturally small nose.

Facial symmetry has been found to increase ratings of attractiveness in human faces. 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.

Facial symmetry is also a valid marker of cognitive aging. Progressive changes occurring throughout life in the soft tissues of the face will cause more prominent facial asymmetry in older faces. More symmetrical faces are also judged to be lower on neuroticism but higher on conscientiousness and agreeableness (asymmetrical faces were rated as less agreeable than normal ones, but the more symmetrical were again rated as somewhat less agreeable than the normal). More symmetrical faces are also more likely to have more desirable social attributes assigned to them, such as sociable, intelligent or lively.

Sexual selection is a theoretical construct within evolution theory. According to sexual selection, mate choice can have profound influence on the preferred features. Sexual selection can only influence features that potential mates can perceive, such as smell, audition (e. g. song) and vision. Such features might be reliable indicators of hidden fitness parameters such as a good immune system or developmental stability. It has been argued that more symmetric faces are preferred because symmetry might be a reliable sign of such hidden fitness parameters. However it is possible that high facial symmetry in an individual is not due to their superior genetics but due to a lack of exposure to stressors, such as alcohol, during prenatal development. It has been found that more symmetrical faces are rated as healthier than less symmetrical faces. Also, facial asymmetry was found to be correlated with physiological, psychological and emotional distress. Some evidence suggests that face preferences in adults might be correlated to infections in childhood.