Definition

Variation in the growth of children occurs in two dimensions. The easiest to understand is the simple variation in height at any given age including full maturity. Less obvious is the variation that occurs in the timing of events during the growth process such as the age at which puberty commences or adult height is reached: this is usually referred to as tempo.

In the case of boys, the average age of attaining adult height is 18 years but this may vary by as much as three years in either direction. Thus a perfectly healthy boy may not stop growing until 21 years of age and this would be an example of growth retardation (Preece, 1998). In the case of girls, the same degree of variation is seen, but, on average, adult height is achieved some two years earlier than for boys.

Once the child reaches mature height, tempo has no more effect and the variation in height amongst adults is not dependent upon it. The major influence on adult height, assuming the individuals are all healthy, is heredity with a typical correlation coefficient between the mid-parental height and the height of the subject being 0.7 (Tanner, 1966). This is in keeping with a polygenic mode of inheritance for stature. During childhood the hereditary element still applies but variation in tempo will also contribute to the total variance in height.