Intrapair facial differences in twins.

Annual serial records in the form of facial contour maps were examined for 18 like-sexed twin pairs of near equal zygosity distribution. Zygosity diagnosis was based primarily on hematological reports for 26 of the 36 children and the remainder were diagnosed on a basis of the concordance or discordance of various physical characteristics: standing height, finger print ridge count, tooth size, and hair and eye colour. Thirteen facial parameters were measured on 274 maps. After age correcting and three-point smoothing, more than 1,150 intrapair differences of individual facial parameters were measured. In general, the dizygotic twin pairs had the larger mean intrapair differences in facial parameters and the monozygotic twin pairs had the smaller intrapair mean differences. The more important facial parameters for distinguishing the two groups were identified and used to calculate a "facial similarity index".