Fetal experience determines some of the characteristics of human adults. Well-nourished mothers have offspring who are adapted to affluent conditions; mothers on a low level of nutrition have offspring who are adapted to lean environments. If the mother's forecast of her offspring's future environment is incorrect, the health of her offspring may suffer severely. The developmental plasticity that accounts for the ill health of humans who are living in conditions of rapid economic change is commonplace in biology. Understanding the evolutionary background sets the developmental origins of ill health in humans in context and has profound implications for public health.