Why do children born at the start of the academic year do better at school than those born later? Using innovative techniques, we were able to show that differences arise primarily because of the age at which children sit the national achievement tests. Our findings contributed to the decision by Ed Balls, then Secretary of State for Children, Schools and Families, to ask Sir Jim Rose to consider the needs of summer-born children in his review of the primary curriculum. The results were also widely reported by the media, contributing to greater public awareness of the issue.