Mental retardation and subnormality in a birth cohort of 12,000 children in Northern Finland.

The occurrence of mental retardation and subnormality were investigated in a one-year birth cohort (N = 12,058) from two provinces in Northern Finland; 96% of the children were followed-up to the age of 14. Of these children 253 (2.2%) were unable to go to an ordinary school and 449 (3.8%) were attending an ordinary school in a class lower than that appropriate for their age. The cumulative incidence of severe mental retardation (IQ less than 50) was 7.4 per 1,000; for mild mental retardation (IQ 50 to 70), 5.5 per 1,000; and for mental subnormality (IQ 71 to 85), 13.4 per 1,000. The corresponding prevalence rates were 6.3, 5.6, and 13.7 per 1,000. The total cumulative incidence of children with IQ less than 86 was 27.0 per 1,000 and the prevalence 25.6 per 1,000. There were 39 cases with Down syndrome, 3.2 per 1,000.