The study originated with all the babies with Down's syndrome born in the year December 1963–November 1964 who lived in the county of Surrey (less the borough of Croydon) and in one area of southeast London (then the boroughs of Camberwell and Lewisham). Fifty-four babies, 25 boys and 29 girls, were referred to and visited by the writer. Forty-five were living at home, nine (three boys and six girls) in various foster homes. Soon after the study began it was decided to include a control group and each home–reared child was matched – for sex, age, and social class – with a non-disabled baby. This sample of non-disabled babies was obtained with the cooperation of the Statistical Division of Somerset House, who, as each baby with Down's syndrome came into the study, supplied names of babies of the requisite sex and social class living within 15 km of the psychologist's home in Surrey. These families were then contacted by their health visitors, who asked for their agreement to a visit from the psychologist. No family refused.

Social class distribution

The family of each home-reared child with Down's syndrome was classified according to the Registrar General's Classification of Occupations (1960) at the outset (age 4) and again at 11 and 21. The distributions at 4, 11 and 21 years are given in Table 2.1.

At age 4 the home-reared children were almost equally divided (49%/51%) between middle and working class families, with a slight overrepresentation of social classes I and II (41%) which was, however, not excessive for Surrey (33%).