Humans prefer attractive faces over unattractive ones. Our preference for attractive faces exists from early infancy and is robust across age, gender and ethnicity. The quest to define facial beauty either by the size or shape of isolated facial features, for example, eyes or lips or by the spatial relations between facial features dates back to antiquity, when the Ancient Greeks believed beauty was represented by a golden ratio of 1:1.618. Although there is little support for the golden ratio, studies have shown that averaging a group of faces results in a synthetic face more attractive than any of the originals. Furthermore, a sufficiently large increase in the distance between the eyes and mouth of an individual face can make the face appear grotesque. Any individual’s facial attractiveness can be optimized when the spatial relations between facial features approximate those of the average face. However, no evidence to date has confirmed this suggestion.
Two types of alterations can be made to the spatial relations between facial features of any individual face. One may alter the vertical distance between the eyes and the mouth; this alteration results in a change in the ratio of this distance to the face length, which is measured by the distance between the hairline and the chin. The ratio is henceforth referred to as the length ratio. The other alteration is to change the horizontal distance between the pupils; this change alters the ratio between this distance and the face width, which is measured between the inner edges of the ears. This ratio is henceforth referred to as the width ratio.
Using a regression analysis to determine the exact relation between the attractiveness score and length ratio, it is found that facial attractiveness follows a curvilinear function with length ratio. Face with an average length ratio is rated as more attractive than faces with other length ratios. This is further supported by the finding that attractiveness scores for faces without an average length ratio were significantly less than the mean attractiveness score for the faces with an average length ratio.
When an optimally attractive state for an individual face in terms of both length and width ratios is examined, it is found that facial attractiveness follows a curvilinear function with the width ratio. When an individual face’s length ratio is already optimal, the optimal width ratio maximizing its attractiveness is 46. Attractiveness scores for faces without an average width ratio were significantly less than the mean attractiveness score for the faces with an average width ratio. Attractiveness scores for faces without an average length ratio were significantly less than the mean attractiveness score for the faces with an average length ratio indicating preference for an ideal length ratio is independent of the width ratio.
In each individual face, there exists an optimally attractive state in terms of both length and width ratios. When the face’s eye-to-mouth distance is 36 percent of the face length and interocular distance is 46 percent of the face width, the face reaches its optimal attractiveness given its unique facial features. Further, although the absolute level of attractiveness may vary with differences in facial features, the optimal length and width ratios remain constant. These optimal, golden ratios correspond with those of an average face. Critically, this preference for average ratios reflects a true preference for the average and not a regression toward the mean. These results may explain some basic daily observations, such as why some hairstyles can make an unattractive face appear more attractive or vice versa. Changing one’s hairstyle may alter the perceived face length or face width, as well as their related length and width ratios, therefore affecting the perceived attractiveness of the face.
Many experiments on attractiveness involve comparing faces that differ in both facial features and spatial relations, but the presence of features that vary in attractiveness could obscure any effect of variation in feature spatial relation on attractiveness. Also, prior research comparing an average face to individual faces failed to discover the ideal length and width ratios for any individual face because the averaging process tends to not only average the spatial relations between facial features but also smoothes the facial features and skin texture. This smoothing effect could artificially increase the attractiveness of the average face, obscuring the effect of average spatial relations on facial attractiveness.
Identifying the optimal length and width ratios for individual facial beauty have attracted a tremendous amount of pursuit, but yet eluded discovery for centuries. Furthermore, the present findings suggest that although different faces vary greatly in absolute attractiveness, for any particular face, there is an optimal spatial relation between facial features that will reveal its intrinsic beauty.
It should be noted that the optimal spatial relations found can also coexist with preferences for sexually dimorphic features. A woman who has large lips, suggesting a strong mating potential, with average length and width ratios will always be more attractive than a woman with narrow lips and average length and width ratios. It is unknown, however, whether the preference for average length and width ratios is stronger than the desire for a pronounced sexually dimorphic trait. In other words, a woman with large lips and unattractive length and width ratios may or may not be preferred to a woman with narrow lips and ideal length and width ratios. Future research is necessary to assess the nature of this trade-off.
By definition, eye-mouth-eye angle involves both horizontal and vertical components. The preference for an average length ratio is independent of the width ratio. Therefore, it is important to note that despite the similarity between the two measures, they may actually measure two very different aspects of the face. While eye-mouth-eye angle provides information on the spatial relations between internal facial features, it also assesses the relation between the internal features and the external facial contour. Since faces are perceived holistically, it is important to consider the facial elements in the context of the whole face. It is possible for the length and width ratios to vary, while eye-mouth-eye angle stays the same, and vice versa. In the context of the whole face length ratios and width ratios appear independent, but within the localized area of the eyes and mouth, there may be an interaction between length and width.
Why should we find a face with an average length and width ratio attractive? Two existing theories provide explanations at two different levels. At the evolutionary level, it has been suggested that humans prefer to reproduce with other healthy mates. Generations of healthy mate selection may act as an evolutionary averaging process. This process leads to the propagation of healthy individuals with physical characteristics, including faces that approximate the population average. As a result, we are biologically predisposed to find average faces attractive. At the cognitive level, it is well established that after exposure to a series of exemplars from one object category, we form a prototype, that is to say, an average for this category. One robust consequence of prototype formation is that we find the prototype more attractive than any individual category members because the prototype is easier to process. Due to this same cognitive averaging mechanism, the average face is perceived as more attractive than any individual face. It is suggested that while the two theories provide different levels of explanation, they may work together to account for our preferences for the optimal length and width ratios for facial beauty. The evolutionary process predisposes us to find average length and width ratios attractive. The cognitive process prescribes what the average length and width ratios are by averaging the ratios of individual faces we have encountered to date.