More than 1000 schoolchildren on St. Helena were studied to determine factors associated with suboptimal growth. Disease was demonstrable in only a small minority, but social factors were of prime importance. In particular, family size was found to be a dominant factor, over-riding the associated effect from overcrowding. The implications of this findings are discussed, and the concept of a "threshold of coping" developed. It is suggested that failure to achieve optimal growth may be a particularly useful index of the points of stress in the child-rearing patterns of society.