Physical appearance may be a window to genetic fitness It is said that beauty is in the eye of the beholder, but personal preferences do not explain the near-universal appeal of, say, Marilyn Monroe or Angelina Jolie. Not everyone would agree that they are perfect but there's no denying their better-than-average looks. Physical beauty is more objective than we choose to admit. Studies in modern science have supported this black and white perspective on beauty. Facial features, for example, are usually more pleasing when they are in proportion. Case in point: Nicole Kidman's large prosthetic nose in The Hours transformed her from a swan into a duckling. Features such as large eyes and full lips can enhance a person's looks, but beauty generally lies in average proportions. The question is why we find these beautiful. According to evolutionary biologists, it comes down to our animal instincts. That is to say, our perception of beauty has been programmed into our brains through natural selection, or Charles Darwin's theory of the survival of the fittest. Physical beauty is a visual indication of how fit or healthy one is genetically and, through centuries of conditioning, our beauty preferences are thus governed by an evolutionary need to find healthy and fit partners. If physical beauty is a window into our fitness, then it is easier to understand why some physical attributes are almost universally attractive. In her book Survival of the Prettiest, Nancy Etcoff explains how, 'in a world of large and small noses, almond eyes and round eyes, oval faces and round faces, overbites and underbites, the eye takes its sums, divides by the numbers and arrives at a mean value'. Apparently, our ability to calculate the 'beauty of averages' is innate. From a biological perspective, this makes sense. In the natural world, average proportions often indicate good health and good design. Birds with the most average wing span have the best lift-off and control of flight, whereas birds with proportionately longer or shorter wing spans are often killed during storms. Similarly, according to statistics, babies that are larger or smaller than average at birth have less chance of survival. From an evolutionary standpoint, the average size and proportions of facial features may be a visual indication of overall good health. But the 'beauty of averages' theory has a few holes. There are instances when exaggerated facial features can make a person distinctively beautiful. Supermodels and celebrities are prime examples. Angelina Jolie's lips are large, yet she is beautiful enough to make Brad Pitt swoon. Might it be that she simply oozes sex appeal? Charles Darwin would say yes - that beauty is bait for sex. The famous biologist theorised that animals are motivated by their desire to propagate and carry on their genes. In other words, they choose the best mate possible for the greatest chance of reproductive success. Modern biologists and psychologists have applied Darwin's theory of sexual selection to the nature of beauty. Certain physical features in women act as visual cues of reproductive fitness, and an exaggeration of these would therefore bump them even higher up the hotness scale. Such visual cues are therefore the result of natural selection, a surefire way to weed out less genetically fit sexual partners. The attractiveness in extreme facial features is found in women more often than in men. A study conducted by British psychologist David Perett, author of a paper called Facial Shape and Judgments of Facial Attractiveness, revealed that women who were rated the most beautiful had thinner jaws, larger eyes relative to their faces and shorter distances between their mouths and chin compared with average-looking women. These traits are exaggerated markers of both femininity and youthfulness. Studies by other scientists have revealed that fuller lips and smaller noses are also traits commonly seen in women considered beautiful. This suggests that beauty detectors for women are actually detectors for youth and femininity, two signs of good reproductive health. As the optimal child-bearing age is between the late teens and early 30s, some scientists believe equating a woman's beauty with youth is a result of natural selection: that men choose mates who score high on the fertility chart. Women's bodies might also give off visual cues of their reproductive fitness. 'The small female waist, poised between rounded breasts and hips, has an ephemeral beauty,' writes Etcoff in her book. Some argue that large breasts have evolved as deceptive signals to attract men, mimicking ample supply of nutrients for their future offspring. The waist to hip ratio seems to have more importance than absolute circumference numbers. If men are looking for fertile mates, this comes as no surprise. A cross-cultural study by Devendra Singh of the psychology department at the University of Texas in the United States revealed that men are most drawn to slender waists and round, child-bearing hips. Men, too, are judged by their physical appearance, although visual cues are harder to quantify. Simply put, many women prefer men who display signs of masculinity - think of Brad Pitt's large, chiselled jaw line. Some studies have found correlations between a large jaw and high levels of testosterone, a male hormone that indicates good health in men. Deep-set eyes, prominent chins, facial hair and broad shoulders are also considered attractive and a sign of strength. However, exaggerated masculine features are not generally considered attractive. Women prefer a balance of dominant and soft features - like large round eyes and wide smiles - to indicate men's 'sensitive side'. Men who are both powerful and masculine and sensitive and kind provide the optimal balance of security as potential life-long partners. Survival of the Prettiest does not paint a pretty picture of our very primal evolutionary needs. For a species that has the capacity to think, feel and learn, evolutionary theories about beauty may come across as insensitive and primitive. But the next time I widen my eyes with mascara or plump my lips with gloss, I might just be reminded of them.