Profile comparisons of physical growth for monozygotic and dizygotic twin pairs.

Three different methods of analysing similarity of physical growth curves within MZ and DZ twin pairs are compared. The concordance of the curves for each pair can be measured by either a multiple regression coefficient (R2) or the square root of the sum of squared differences between corresponding deviation points (D). The latter seems to be a more appropriate measure of profile contour similarity, mainly because the distance between curves is allowed for. An intra-class correlation based on a longitudinal analysis of variance seems to be a useful measure of average profile similarity for comparing groups of individuals. The distributions of the D measures, calculated for all MZ and DZ twin pairs, show MZ pairs to have significantly smaller D-values for both height and weight, on average, than DZ pairs. The intra-class correlations also show a higher concordance in developmental profile for MZ in comparison to DZ.