Intra-pair similarity in physical growth of opposite-sex twin pairs during puberty.

A comparison has been made of within-pair similarity in physical growth for opposite-sex twin pairs and pairs of unrelated controls attending the same classes as the twins. Height and weight measurements were originally collected for 97 pairs of twins and controls from 10 to 16 years of age for the girls and from 10 to 18 years for the boys. The twins showed significantly smaller within-pair differences than the controls for both height and weight during the whole investigation period. Within-pair correlations for height and weight were negative for the controls and moderately positive for the opposite-sex twins. The weight correlations for the twins were lower than for height from 11 to 14 years of age. At age 15 the height and weight correlations tended to be of the same magnitude due to a drop in within-pair similarity for height at this age. Profile similarity in physical growth has been studied both for each pair individually (D-values) and for the whole group of twins and controls (analysis of variance). Both comparisons showed both the twins and the controls to be very dissimilar in growth profiles during puberty. Onset of puberty was estimated by age at peak height velocity (PHV) and peak weight velocity (PWV). The average within-pair difference for the controls was shown to be a little more than two years and for the twins a little less. The small and non-significant differences between the groups lead to the main conclusion that sex differentiation at puberty exerts a very powerful influence overshadowing the hereditary and environmental similarities existing for the twins.