Social class differences in mental retardation and subnormality

Social class and regional differences in mental retardation were studied in a birth cohort of 12,000 children followed up until the age of 14. The incidence of severe mental retardation IQ less than 50 was significantly higher in farming families and in less developed areas, which is in agreement with the fact that a greater part of the population in these areas belong to social class IV and farmers. The obvious explanation for the higher incidence of severe mental retardation among farmers is an excess of older mothers in this group. In the group of severe mental retardation, paternal unemployment was also statistically, significantly more frequent than among the others. Mild mental retardation, IQ 50-70, was significantly higher in all classes other than I + II and mental subnormality, IQ 71-85 was higher in social classes III and IV. The other less favourable social conditions, which were statistically more frequent in the families of the mentally subnormal, were that: the father had died, was unemployed, on sick leave or receiving a pension and the mother was not living at home, was unemployed, or was on sick leave or receiving a pension. The incidence of mental subnormality was significantly higher in more developed areas, in spite of the fact that the members of social class IV were less and those of classes I + II more numerous than elsewhere.(ABSTRACT TRUNCATED AT 250 WORDS)