Mental retardation in Norway: prevalence and sub-classification in a cohort of 30 037 children born between 1980 and 1985

OBJECTIVES: We wanted to determine the prevalence and subcategories of mental retardation (MR) defined as IQ < or = 70 in Akershus county, which rated by average yearly income, had the second highest socioeconomic status (SES) in Norway. METHODS: The study population consisted of 30 037 children born between 1980 and 1985. Cases were ascertained from multiple sources and psychometrically assessed, predominantly with the Wechsler tests, which were standardized in Norway in 1978. The cases were divided into four groups: profound (IQ < 20), severe (IQ 20-34), moderate (IQ 35-49), and mild (IQ 50-70) MR, according to the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV). The median age was 10.8 y. The frequency and degree of parental consanguinity were registered, and the average inbreeding coefficient was calculated. RESULTS: Altogether, 185 had IQ < or = 70, giving a prevalence of MR of 6.2/1000. Prevalences for profound, severe, moderate, and mild MR were 0.8, 0.4, 1.5 and 3.5/1000, respectively. In two, possibly three, cases the parents were second cousins, giving an average inbreeding coefficient of 20-30 x 10(-5). CONCLUSIONS: The low prevalence of MR, particularly mild MR, could partly be explained by high SES, old standardization of IQ-test, and low inbreeding coefficient. The proportion of profound MR was considerably higher than estimated by DSM-IV.