From the start of universal schooling there has been dissatisfaction with the outcomes of primary education and disagreement about why some children make much slower progress. Contrary to public perceptions, the differences between children in the same class taught by the same teacher are greater than differences between schools or countries. This inaugural lecture considers the factors that may be influential, drawing on findings from research projects funded by the ESRC and the Nuffield Foundation. Recent accounts in developmental psychology emphasize reciprocal relationships between factors - for example a child's cognitive skills may affect their mathematical development and mathematical education can enhance the development of cognitive skills. Understanding such reciprocal relationships is required to advance developmental psychology and improve the basis for educational policies.