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. 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.
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.