In the search for a causal relationship between undernutrition and poor development in children the functional isolation hypothesis maintains that lack of dietary energy leads to reduced activity levels which in turn leads to reduced exploration and developmental delays. Early investigators described reduced activity in severely undernourished children but systematic observations were not done. They can be divided into cross-sectional studies nutritional supplementation studies and studies involving the reduction of energy intake. In Guatemala 69 poor children aged 2-6 years old were observed on 2 weekdays (Torun 1990). Mildly malnourished children (n 21) spent less time in more vigorous activities than the adequately nourished children (n 43). Of the cross-sectional studies reviewed conclusions cannot be drawn. Spurr & Reina (1987) examined the effect of nutritional supplementation on the energy expended in activity in undernourished Colombian girls aged 8-11 years. After more than 4 months no increase was found in spite of an increase in growth velocity. Two studies were carried out at a clinical center in Guatemala on boys who had recovered from severe malnutrition. Both these studies suffer from small sample sizes and it is uncertain whether the response to reduced intake can be extrapolated. In Jamaica 78 stunted children aged 12-24 months were randomly assigned to 1 of 3 treatment groups: one group receiving nutritional supplementation another receiving both supplementation and psychosocial stimulation and controls who received neither intervention. 26 matched non-stunted children were also studied. Reduced activity levels of stunted children were related to locomotor development at one age but not at another. Supplementation improved development but did not affect activity and the control stunted group continued to have poor developmental levels in spite of increasing activity levels. These findings only partly support the functional isolation hypothesis as far as stunted children are concerned. It is likely that the childs curiosity may be more important than gross motor activity in influencing development.