Three-dimensional assessment of the relative contribution of genetics and environment to various facial parameters with the twin method.

The purpose of this prospective study was to help distinguish the relative contributions of heredity and environment to the external facial features and to assess the relative strength of the genetic influence on anteroposterior and vertical facial parameters, by using 3-dimensional optical surface scanning and the inferential twin method. The subject group consisted of 10 pairs of monozygotic twins (5 male, 5 female, mean age 11.9 years), 10 pairs of same-sex dizygotic twins (3 male, 7 female, mean age 12.1 years), and 6 mixed-sex dizygotic twin pairs. Two methods of analysis were used on the 3-dimensional scans. Interlandmark measurements of 28 facial parameters were recorded, and the mean intrapair differences for each parameter were compared between the monozygotic and dizygotic groups. The second method used the analysis of surface shape for all twin pairs. This technique characterizes the face by using 9 surface shapes distinguished visually by color, allowing a qualitative description of the differences between monozygotic and dizygotic twin pairs. Analysis of the 28 facial parameters showed significant ( P< .05) genetic determination for midfacial parameters, especially left eye width, intercanthal width, nose height, and nose width. Analysis of surface shape showed the strongest genetic determination for a triangular area of the midface encompassing the orbital rims, intercanthal area, and nose. The concordance for vertical and anteroposterior facial parameters was found to be greater in monozygotic twins than in dizygotic twins. The concordance for vertical facial dimensions of the middle and lower anterior parts of the face was found to be greater than that for anteroposterior facial dimensions in monozygotic twins.