SummaryTriceps and subscapular skinfold thicknesses were measured in 222 pairs of like-sex twins (78 monozygotic and 144 dizygotic) aged 3-15 years. Log transformations of the measurements were standardized for age and sex and the results used to estimate heritability-that is, the proportion of total variation determined by genetic factors. The overall contribution of non-genetic familial effects was small. There were appreciable differences in heritability between limb and trunk fat and between the sexes and at different ages. Over the age of 10 heritability was high for both sites in boys and girls. In younger children environmental factors contributed more to the variation. IntroductionThe separation of heredity and enviromment in the determination of body fat has proved difficult for want of suitable material and sufficient data. The use of weight alone to measure fatness in humans is uninformative, and weight-for-height indices do little to improve the information because of the large differences in body proportions between people of the same height and sex. Skinfold thicknesses correlate well with total body fat in both adults' and children. 2 One study of monozygotic twins brought up together and apart depended only on measurement of weight.3 Another reported parent/child correlation coefficients of weight and skinfold thickness in children up to the age of 7 years; for weight

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The complexity of determining the relative importance of genetic and environmental influences in the variations observed in body build is considerable. The availability of triceps and subscapular skinfold measurements collected on identical and fraternal twins during childhood, together with suitable standard measurements, has provided an opportunity to do this. The proportion of variance for a characteristic which is due to genetic variation can be calculated from the difference between correlation coefficients of the characteristic observed in monozygotic and dizygotic twins, if it is assumed that the within-pair differences due to environmental factors would be similar for both. Triceps and subscapular skinfold thickness have been measured on 78 pairs of identical twins (38 male, 40 female), on 144 pairs of fraternal twins (67 male, 77 female), and on 117 pairs of fraternal twins of unlike sex. There were marked differences between the two sexes and between the two sites: age also considerably affected the results. In children under the age of 10 years environmental influences accounted for a large proportion of the variation of limb fat in both sexes and body fat in girls. In older children genetic factors accounted for a very large proportion of the variation in both types of fat in both sexes.

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