A serial study of normal facial asymmetry in monozygotic twins.

Asymmetry of the soft tissues of the face was measured by comparing three pairs of bilateral facial parameters connecting the landmarks external canthus, tip of nose and angle of mouth. The subjects were six pairs of monozygotic twins (eight girls and four boys) in the age range 8-19 years. A clinical contour mapping technique known as short-base stereophotogrammetry provided life-size facial maps on which the parameters were measured in three dimensions. The study was based on annual serial records over an average period of 9 years for each pair, and was almost entirely longitudinal. The asymmetry in millimetres was given a positive or negative sign, depending on whether right or left side dominated. Asymmetry in the facial parameters was found to be very small, amounting at most to a few millimetres, and was not much larger than the measuring error of the method. Nevertheless, asymmetry was statistically significant (p < 0.05) in 12 out of 60 indicators, and of these seven were at p < 0.01 level. Asymmetry could not be related to twin zygosity, adolescence or age.