Children from families belonging to the high or middle socio-economic groups in any country are on average larger in body size than their counterparts in the lower economic groups. High income and high educational level imply not only better nutrition but often also better child care and better use of medical and social services. Differences between socio-economic classes in height and weight are found in developed as well as in developing countries. In the British National Child Development Survey, a nationwide sample of children consisting of all those born in one week of March 1958, an overall difference of 3.3 cm was reported between seven years olds from the professional and managerial classes and those from the unskilled, manual working class. Similarly in the U.S. national sample of children measured by the National Centre for Health Statistics, children aged 6 to 11 were some 3 cm taller in the rich families than in the poor ones. At the University of Paris, the tallest students were children of parents in intellectual professions while the shortest and heaviest from worker and peasant families. Reports of socio-economic class differences in growth from developing countries including India show the very great differences in height and weight of 5 years olds in upper and lower social classes that exist in many cities.
These differences are compounded by differences in height and tempo according to number of sibs in the family. First born children are somewhat taller than later born children with the same number of sibs, since they have had a period of being an only child. The more mouths to feed, it seems or simply more children grow. This difference is solely of tempo, because children do not differ systematically according to birth order when they are fully grown. All the differences in social class may not be of direct environmental origin, however classes are to some extent endogamous, and movement from one social class to another in some countries is linked with size as well as ability.
In Belgium, young men who were moving up the scale occupationally (i.e. entering a more prestigious and better rewarded occupation than their fathers) were larger, healthier and scored higher on intelligence test than those who stayed in the same or equivalent occupation. The downwardly mobile showed approximately the opposite picture