The psychoanalysis of children began to flourish in the 1920s. In exactly the same period, the technique of intelligence testing also began to expand. Yet the relation between these two theoretical advances is often overlooked and misunderstood. This article focuses on the British context and considers why it is vital to consider the history of child psychoanalysis in relation to intelligence testing. The first half considers the growth of child psychoanalysis from the 1920s and reflects on how psychoanalytically informed thinkers such as Jean Piaget, Susan Isaacs and Donald Winnicott considered children's intellectual capacities in relation to emotional engagement. The second half considers major changes in approaches to mental health and ‘mental deficiency’ in the late 1950s, and explores how this led to a mounting criticism of psychoanalytic theories of ‘autistic’ and ‘psychotic’ thought. The article concludes with a reflection on how political change in the 1970s and 1980s influenced new models of child development and encouraged new psychoanalytic work.