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Mirror, Mirror on the Wall, Who's the Most Symmetrical of Them All?:

Professors conduct research on a biological constant of beauty.

by Larry Walsh

Beauty sells cars, magazines, clothes, 500 million dollars of lipstick each year and just about anything else you can name. In 1998, the pursuit of beauty resulted in over one million Americans undergoing cosmetic surgeries.

And no wonder: beauty not only sells, it pays. There have been studies to show that good-looking people receive more attention from their parents, teachers, and friends, and may even get more promotions and better earnings on the job.

Societies throughout history have been preoccupied with beauty, but the prevailing wisdom has been that physical beauty itself is arbitrary. That is, like the full-figured women of Rubens' paintings, what is considered beautiful in one culture at one moment of time would not be deemed attractive in another.

However, the pioneering research of Randy Thornhill, UNM Regents' professor of biology, and Steven Gangestad, professor of psychology, has shown that beauty may not be entirely skin deep. There are a number of biological constants that transcend culture, ethnicity, and advertising campaigns. One of these is bilateral symmetry, which is how well your left side matches your right.

Thornhill's and Gangestad's investigation of human attractiveness and its role in the evolution of human mating began interestingly enough. In 1990, Thornhill had just returned from Japan and Germany where he had been studying scorpion flies. His research showed that female scorpion flies were attracted to males with symmetrical (same size) wings.

Biologists consider bilateral symmetry to be a marker of developmental stability. The tendency in the natural selection of any animal is to develop perfectly matching eyes, wings, legs, arms, feet, ears, etc. However, balanced development can be thrown off course by mutations, toxins, and disease. More resilient individuals can more readily overcome these setbacks and better maintain their symmetry. Such resiliency has a genetic basis and can in part be inherited. Consequently, bilateral symmetry can be seen as a health certification, and as such, a logical reason for females to select males as mates, at least in the case of scorpion flies.

From the scorpion fly research, Thornhill came to question whether bilateral symmetry played any role in human attraction. In collaboration with Karl Grammer of the University of Vienna, Thornhill examined facial attractiveness and bilateral symmetry. Male and female college students were asked to rate the attractiveness of a series of photographed faces. The faces were then measured as to their bilateral symmetry on seven different facial features: the placement of their pupils, innermost eye corners, outermost eye corners, cheekbones, and the outer edges of the nose, mouth, and jawbone.

By drawing lines between these paired features and marking their midpoints, the researchers were able to develop an overall index of asymmetry. When these data were correlated to the ratings of attractiveness, they showed that symmetry is one of the factors that influences the perception of beauty. Several other studies suggest that this correlation may hold true for all human beings.

In order to explore attraction and human behavior, Thornhill then teamed up with Gangestad, an evolutionary psychologist who had been studying neurological symmetry through the development of right- and left-handedness. Evolutionary psychology uses a biological approach to study human behavior. It assumes that many features or traits were adaptations specifically designed to solve particular problems. Through a process of reverse engineering, the specific design of an adaptation can be used to determine its function and shed light onto the evolutionary history of human behavior.

Thornhill and Gangestad first extended their initial research by measuring the body symmetry of close to 1,000 male and female college students. These measurements included the breadth of the feet, ankles, hands, wrists, elbows, and the length and width of the ears. This series of studies established that body symmetry was directly correlated to attractiveness. This is not to say that people look at how well your ankles match, but rather, how well your ankles match is a reflection of your overall developmental health and attractiveness.

However, because of the advances in modern medicine, a direct correlation between health and symmetry has been difficult to establish. One 1997 study (Manning, Scutt, Whitehouse, and Leinster) has suggested that women with asymmetrical breasts have a higher rate of breast cancer. A Thornhill and Gangestad study in the West Indies has indicated that more symmetrical men have a low incidence of diseases. These and other studies have led Thornhill and Gangestad to conclude that while bilateral symmetry might not distinctly be a certification of health in today's evolutionary environment, it has been a strong marker of health throughout human evolutionary history.

Since bilateral symmetry may confer a potential benefit to offspring, it then becomes important to examine how it affects sexual behavior. As part of a study on body symmetry, Thornhill and Gangestad gathered the participants' sexual history. They found that more symmetrical males tended to have more sexual partners in their lifetimes. These males were more likely to engage in sexual activities outside of their primary relationships, and were less likely to invest time and energy in maintaining those relationships.

On the other hand, they found that more symmetrical females did not have a larger number of sexual partners than their less symmetrical counterparts.

An interesting aspect of this research was the behavior of women (both symmetrical and asymmetrical) in relationship to more symmetrical men. Women showed a marked preference for symmetrical men for short-term relationships.

Within an evolutionary framework, biologists have long argued that the differences between male and female sexual behavior is directly connected to reproductive success. Since women can only have one child per year, their success is linked to making sure that child survives; therefore, women tend to select mates who will invest in raising offspring. Men have the best chance of success in having multiple partners to produce the largest number of offspring, which in turn reduces their investment in any one child.

Thornhill's and Gangestad's research, however, argues that both males and females have mixed sexual strategies. Presented with more opportunities, more symmetrical males opt for more partners, with a corresponding decrease in investment in relationships. In contrast, asymmetrical males have found that their best chance for reproductive success lies not in multiple mates, but in investing in their relationship and offspring.

Women, on the other hand, tend to choose men (symmetrical or not) who will invest in the relationship and the raising of children as long-term mates. But when the possibility of conception is at its highest, women are more likely to select more symmetrical males for short-term relationships, thus providing better genetic inheritance for their offspring. These strategies may be conditional. When women have more access to resources (and therefore less need of a long-term relationship) or when diseases are prevalent (making resistance inheritance more important), women may tend to select more symmetrical males for all relationships.

Beauty is more than skin deep. It has a genetic and developmental basis which influences perceptions of beauty and human sexual behavior. Gangestad's and Thornhill's research has helped shed light on the biology of beauty, courtship, and mating.

A regular contributor to Quantum, Larry Walsh is a freelance writer and Peabody Award winning television producer.

Copyright Quantum. All rights reserved. Reprinted at with permission. recommends the following books from


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