Proportion of body mass to body structure

The Body Mass Index (BMI) is another important universal determinant to the perception of beauty. . The BMI refers to the proportion of the body mass to the body structure. However, the optimal body proportion is interpreted differently in various cultures. The Western ideal considers a slim and slender body mass as optimal while many historic cultures consider an embonpoint or plump body-mass as appealing.

In either case the underlying rule applied in determining beauty is the BMI, and hence displays how cultural differences of beauty operate on universal principles of human evolution. The phenotype of one's own mother during the early years of childhood becomes the basis for the perception of optimal body mass index (BMI).

The slim ideal does not consider an emaciated body as attractive, just as the full-rounded ideal does not celebrate the over-weight or the obese. The cultural leanings are therefore just social emphasis on specific phenotypes within a parameter of optimal BMI.

The attraction for a proportionate body also influences an appeal for erect posture.

Prototypicality as beauty Besides biology and culture, there are other factors determining physical attractiveness. The more familiar a face seems, the more highly it is judged, an example of the mere exposure effect. It is seen that when many faces are combined into a composite image (through computer morphing), people find the resultant image as familiar and attractive, and even more beautiful than the faces that went into it. One interpretation is that this shows an inherent human preference for prototypicality. That is, the resultant face emerges with the salient features shared by most faces, and hence becomes the prototype.

The prototypical face and features is therefore perceived as symmetrical and familiar. This reveals an "underlying preference for the familiar and safe over the unfamiliar and potentially dangerous". However, critics of this interpretation point out that compositing computer images also has the effect of removing skin blemishes such as scars, and generally softens sharp facial features.

Classical conceptions of beauty are essentially a celebration of this prototypicality. This shows the importance of prototypicality in the judgment of beauty, and also explains the emergence of similarity of the perception of attractiveness within a community or society, which shares a gene pool.