Between c.1945 and 1965 across the West special education has grown and differentiated substantially. In the Netherlands this expansion ran parallel to the academic recognition and rapid development of the study of learning disabilities. How are these two processes related? This article argues that in this country child science and special education have mutually stimulated each other’s growth and development. The creation of new categories of special-needs children brought about a climate in which the study of learning disabilities and their treatment could flourish. This, in turn, produced further differentiation between children with learning difficulties. Soon problems of identification and categorisation of mentally ‘subnormal’ children proved too complicated to rely on intelligence testing and medical-psychological diagnosis alone. Educational prognosis, based on long-term observation and all kinds of testing, became the key to a child’s future at school and educationists instead of psychologists became the foremost keyholders.