figure 3-7 (A) Peak height velocity-centered plots of the yearly increments of two boys together with the cross-sectional mean of the two curves. (B) PB1 velocity curves of two boys together with the mean-constant curve. (Data from Chrzastek-Spruch's unpublished report on the Lubin Longitudinal Growth Study.53)
sex differences in growth
It is well known that women are on the average smaller than men for all linear body dimensions, in particular height, sitting height, and leg length.27 Although some differences between boys and girls may already be present at birth, they remain, in general, small until the time that girls start their pubertal growth spurt. Because of the 2-year difference in age of onset of the pubertal spurt, 11-, 12-, and 13-year-old European girls are, on average, taller and heavier than the boys of the same age.55 Some findings suggest a positive correlation between the amount of sex differences in adult size and the sex average for size.56,57 However, Eveleth58 found a relatively larger sex difference in adult stature in Amerindians, who have a relatively smaller adult size. Similar findings were reported for Asian Indians.59 Eveleth postulated that genetic factors probably play an important role in establishing mature size and sex differences, although it is conceivable that boys are treated better in these societies than girls, allowing them to better express their genetic potential than girls.
Most of our knowledge on sex differences in growth is derived from cross-sectional data, which allow us to estimate fairly accurately the sex differences during infancy, childhood, and adulthood, as well as the points of intersection between the male and female average growth curve. However, for reasons explained already, cross-sectional data poorly reflect individual growth and longitudinal data is needed to understand the manner in which sex differences in size arise during the growth process.34 Using mean-constant curves, allowing estimation of the typical male and female curves in the population, one can analyze the dynamics of the sex dimorphism in human growth. As an example, Figure 3-8 shows the sex differences in the mean-constant curves for Belgian boys and girls. Note that takeoff is pointed out by a black dot. We consider prepubertal growth as the size achieved up to the age at takeoff, while adolescent growth (or adolescent gain) is the amount of growth achieved between takeoff and adulthood. Figure 3-8 illustrates how the sex difference in adult size (D) can be decomposed into three additive components:
DA: The difference in adolescent gain between boys and girls.
DP: The difference in prepubertal growth; that is, the difference in size at the girls' takeoff.
DT: The amount of growth achieved by the boys between the girls' and the boys' takeoffs.
Using this technique, Hauspie et al.59 analyzed the dynamics of the sex differences in height, sitting height, shoulder width, and hip width in British children. The results are summarized in Table 3-2.
It can be seen that the largest contribution to sex differences in adult height comes from the later onset of the pubertal growth spurt in boys than in girls (DT = 7.9 cm). Sex differences in prepubertal growth (DP = 2.0 cm) and in adolescent gain (DA = 2.0 cm) are relatively smaller. Slightly different values for these components may be found in other populations. In West Bengal children, for instance, Hauspie
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