In my last post, I explained that the standards of beauty are culturally universal and innate. There are three main features that characterize beautiful faces: the geometric feature of bilateral symmetry, the mathematical feature of averageness, and the biological feature of secondary sexual characteristics. They all indicate genetic and developmental health of beautiful people.

Bilateral symmetry. Attractive faces are more symmetrical than unattractive faces. Bilateral symmetry (the extent to which the facial features on the left and the right sides are identical) decreases with exposure to parasites, pathogens, and toxins during development, and with genetic disruptions such as mutations and inbreeding. Developmentally and genetically healthier individuals have greater symmetry in their facial and bodily features, and are thus more attractive.

For this reason, across societies, there is a positive correlation between parasites and pathogen prevalence in and the importance placed on physical in mate selection; people place more importance on physical attractiveness when there are more pathogens and parasites in their local environment. This is because in societies where there are a lot of pathogens and parasites in the environment, it is especially important to avoid individuals who have been afflicted with them when selecting mates.

Averageness. Facial averageness is another feature that increases physical attractiveness; faces with features closer to the population average are more attractive than those with extreme features. In the memorable words of Judith H. Langlois of the University of Texas, who originally discovered that the standards of beauty might be innate, “attractive faces are only average.” Evolutionary psychological reasons for why average faces in the population are more attractive than extreme faces are not as clear as the reasons for why bilateral symmetry is attractive. Current speculation is that facial averageness results from the heterogeneity rather than the homogeneity of . Individuals who have two different copies (or alleles) of a gene are more resistant to a larger number of parasites, less likely to have two copies of deleterious genes, and at the same time more likely to have statistically more average faces with less extreme features. If this speculation is correct, it means that, just like bilateral symmetry, facial averageness is an indicator of genetic health and parasite resistance.

Secondary sexual characteristics. Unlike the geometric concept of bilateral symmetry and the mathematical concept of averageness, the biological concept of secondary sexual characteristics differ for the sexes. For men, features that are considered to be attractive are indicators of high levels (such as large jaws and prominent brow ridges). For women, features that are considered to be attractive are indicators of high levels of (such as large eyes, fuller lips, larger foreheads, and smaller chins). This is probably why women instinctively tilt their head forward and look up (making their eyes and forehead look larger than they are and their chin smaller than it is) when they want to appear attractive. (Think of the way Princess Diana was typically photographed). Similarly, men instinctively tilt their head back (making their jaw appear larger than it is and their brow ridge more prominent than it is) when they want to look attractive. For both sexes, faces that typify higher levels of -typical are considered to be attractive.

Far from being only skin deep, beauty appears to be an indicators of genetic and developmental health, and therefore of mate quality; beauty is a “health certification.” More attractive people are healthier, have greater physical fitness, live longer, and have fewer lower back pain problems (although some scientists dispute these findings). Bilateral symmetry measures beauty so accurately that there is now a computer program that can calculate someone’s facial symmetry from a scanned photograph of a face (by measuring the sizes of and distances between various facial parts) and assign a single score for physical attractiveness, which correlates highly with scores assigned by human judges. A computer program can also digitally average human faces. Beauty therefore is an objective and quantifiable attribute of individuals, like height or weight, both of which were more or less “in the eye of the beholder” before the invention of the yardstick and the scale.