The book reports a clinical and epidemiologic study of the prevalence, distribution, and antecedents of mental subnormality in 8 to 10 year old children living in Aberdeen, Scotland (population 187,000). Utilizing three types of data (differential clinical diagnoses, biological background information, and social characteristics), the study revealed the incidence figure of 104 from the population of 8,274 children in the specified age range (prevalence rate 12.6 per 1,000). Detailed analyses of etiological considerations were conducted in areas of prenatal, birth, and perinatal conditions. Etiologic implications of the social distribution of the subtypes of mental subnormality, characteristics of family considerations in the lower social classes, and an intensive clinical consideration of births resulting in mentally handicapped children were also reported. Appendixes contain data on pupulation characteristics, research methodology, evaluation criteria, and definitions for classification. (RD)