Are human preferences for facial symmetry focused on signals of developmental instability?

Humans find symmetrical faces more attractive than are asymmetrical faces. Evolutionary psychologists claim that our preference for symmetry can be explained in the context of mate choice because symmetry is an honest indicator of the genetic quality of potential mates. These arguments assume that asymmetry in human faces is fluctuating asymmetry (FA), because this form of asymmetry can be revealing of developmental instability. However, no study has yet examined the characteristics of facial asymmetry. Here we provide the first detailed study of the patterns of asymmetry in human faces. We measured asymmetry in 35 facial traits. Although some traits had distributions characteristic of FA, many had distributions that characterize directional asymmetry (DA); on average, both men and women had right hemi-face dominance. For DA traits we used deviations from the mean asymmetry as a measure of developmental instability. Our measures of asymmetry accounted for a moderate proportion of the variance in perceived symmetry. Importantly, only FAs and random deviations from DA contributed to people's perception of symmetry. DA was not important in symmetry judgments. Faces rated as symmetrical were also rated as attractive. Random deviations from DA were weakly related to women's attractiveness judgments of men's faces. DAs did not influence attractiveness judgments. Our data suggest that people focus on aspects of facial asymmetry that may be revealing of developmental instability. Further studies that isolate FA from other forms of asymmetry are required to accurately assess the influence of developmental instability on the quality of individuals and its potential role in mate preferences.