Facial symmetry

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Facial asymmetry
)
While symmetrical faces (leftmost image) are perceived to be attractive, completely symmetric faces (a and b) are disconcerting and are not perceived as normal.

Facial symmetry is one specific measure of bodily

aesthetic traits of physical attractiveness and beauty.[1] For instance, in mate selection, people have been shown to have a preference for symmetry.[2][3]

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.[4] 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.[5]

Directional 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,[6] and inner organs (see axial twist theory), but also from various animals (see Symmetry in biology).

Aurofacial asymmetry

Exaggerated schema of the aurofacial asymmetry as predicted by the axial twist theory.[7] During embryology and development, the face elements (red) are predicted to move toward the center from the left, with respect to the mid-plane between the ears. Source: figure 1c of reference[8]

Aurofacial asymmetry (from

growth.[8][9]

Anatomy and definition

In contrast to fluctuating asymmetry, directional asymmetry is systematic, i.e. across the population it is systematically more often in one direction than in the other. It means that across the population a deviation is more often to one direction than to the other, i.e., there is a

statistically significant bias to one direction. In case of directional asymmetry, most individuals of a species are asymmetric to the same side, even though some individuals can be symmetric, or even asymmetric to the opposite side (cf., e.g., handedness). The relation between directional and fluctuating asymmetry is comparable to the concepts of accuracy and precision
in empirical measurements.

The aurofacial asymmetry is defined as the position of the face (mouth, nose and eyes) with respect to the mid plane of the axis through the ears. The asymmetry is expressed as an angle (degrees), i.e. by how many degrees facial landmarks (e.g. tip of the nose) or pairs of landmarks (e.g. inner corners of the eyes (endocanthions are rotated away from the mid plane between the ears.[8]

Magnitude across the face and development

Average aurofacial asymmetry in 200 adults. A. Red-shaded: significant asymmetry towards the left ear. B. The amount of asymmetry at the level of each arrow in panel A. Source: figure 2 of reference,[8] and data from [10][11]

On average, the aurofacial asymmetry is slightly larger for the eyes than for the nose, as shown by the figure.

The gradual decrease of the aurofacial asymmetry with age until the beginning of adulthood. Source: figure 3 of reference[8] on the basis of data from ref [12]

In humans asymmetric growth leads to a gradual reduction of the aurofacial asymmetry.

adults
.

Theory and evolution

The aurofacial asymmetry was discovered

vertebrates
, but this has never been tested.

The axial twist occurs in the early embryo. Shortly after the

anterior
head region makes a half-turn around the body axis in anti-clockwise direction (looking from tail to head), whereas the rest of the body (except heart and bowels) make a half-turn in clockwise direction. Since the axial twist is located between the ear-region and the forebrain-face-region, it is predicted that the face grows from the left to the midline, as is indeed the case.

Fluctuating asymmetry

Fluctuating asymmetry is the non-systematic variation of individual facial landmarks with respect to the facial midline, i.e., the line perpendicular to the line through the eyes, which crosses the tip of the nose and the chin.

A wide variety of methods have been used to examine the claim that facial symmetry plays a role in judgments of beauty. Blending of multiple faces to create a composite[1][13][14] and face-half mirroring[15] have been among the techniques used.

  • 2 right sides
    2 right sides
  • Original
    Original
  • 2 left sides
    2 left sides

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.[16]

Attractiveness

Facial symmetry has been found to increase ratings of attractiveness in human faces.[1][3] More symmetrical faces are perceived as more attractive in both males and females, although facial symmetry plays a larger role in judgments of attractiveness concerning female faces.[17]

While studies employing the composite faces produced results that indicate that more symmetrical faces are perceived as more attractive,[citation needed] studies applying the face-half mirroring technique have indicated that humans prefer slight asymmetry.[citation needed] Also, studies have shown that nearly symmetrical faces are considered highly attractive as compared to asymmetrical ones.[18] The symmetry of the nose seems to be more important than that of the lips.[citation needed]

Dynamic asymmetries

Highly conspicuous directional asymmetries can be temporary ones.[19] 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.[16]

Facial averageness vs. symmetry

Experiments suggest that symmetry and averageness make independent contributions to attractiveness.[20][21]

Aging

Facial symmetry is also a valid marker of cognitive aging.[22] Progressive changes occurring throughout life in the soft tissues of the face will cause more prominent facial asymmetry in older faces.[16] Therefore, symmetrical transformation of older faces generally increases their attractiveness while symmetrical transformation in young adults and children will decrease their attractiveness.[16]

Personality and Big Five personality traits

Research indicates that facial symmetry is correlated with the 'big-five' model of personality. The five factors are:[citation needed]

A consistent finding is that facial symmetry is positively correlated with extraversion, indicating that individuals with more symmetric faces are also more extroverted.[23][24][25] 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).[26] More symmetrical faces are also more likely to have more desirable social attributes assigned to them, such as sociable, intelligent or lively.[25]

However, the relationship of facial symmetry and the 'big-five' personality model remains somewhat unclear with regard to neuroticism, openness, agreeableness and conscientiousness. Openness and agreeableness appear to be significantly negatively related to facial symmetry, while neuroticism and conscientiousness do not seem to be linked to facial symmetry.

trustworthiness it has been found that the facial muscles become imbalanced when lying.[27]

Evolution and sexual selection

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.[28] 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.[12]

It has been found that more symmetrical faces are rated as healthier than less symmetrical faces.[3][17] Indeed, facial symmetry was found to be positively associated with the perceived healthiness of the facial skin.[29] Also, facial asymmetry was found to be correlated with physiological, psychological and emotional distress.[30]

Some evidence suggests that face preferences in adults might be correlated to infections in childhood.[31]

See also

References

External links