This study was conducted during the course of a doctoral thesis at Cambridge University under the supervision of James Russell, funded by a grant from the Medical Research Council, and written while in receipt of a grant from the Fyssen Foundation, Paris. Warm thanks go to the pupils and staff at the following schools for children with autism: Broom-Hayes, Westward Ho!; Hope Lodge, Southampton; Peterhouse, Southport; Portfield, Christchurch; Priory, Doncaster. Many thanks also to the pupils and staff at the following schools for children with MLD: Horizons, London; Lady Adrian, Cambridge; Rees Thomas, Cambridge; and to Millington Road and ACE Nursery schools in Cambridge.