The prevalence of mental retardation: a critical review of recent literature

Mental retardation (AIR) is a serious and lifelong disability that places heavy demands on society and the health system. Since the first publication on this topic',theprevalenceofMR has been thoroughly studied for different purposes. Most prevalence studies are designed for the planning of services and establish an 'ascertained' prevalence rate, which is the number of cases officially recorded by the authorities'.The 'true'prevalence rate is the total number of mentally retarded people in a population, whether or not they require services,and is defined by the prevalence of MR a t birth and the mortality rate.For mild mental retardation (MWR, I& 50-70) ' the true prevalence rate is more difficult to estimate than for severe mental retardation (SMR, IQ <50).Very often ascertained prevalence rates are mistaken for true prevalence rates. Moreover, the estimates of both rates are influenced by the design of the study, the assessment criteria used, and the method applied for the identification of case^^-^. As a result, differences in prevalence rates might partly reflect the true variation over populations and partly reveal discrepancies between studies and in the interpretation of the prevalence measure used. For instance, different frequencies are yielded by uses of the organic, psychological and social WHO criteria, which are reflected in impairment, disability and handicaps. The prevalence rates observed range from two to 85 per 1000. According to the WHO, the true prevalence rate of total MR in industrialised countries comes close to 3%".O, but in the United States controversy exists over whether the rate is 1% versus 3%5.7.'0.1', whereas the Scandinavian countries claim that the 1 % figure is their true pre~alence~. '~ . '~ . ' The aim ofthis annotation is to establish valid estimates of the true prevalence rates for SMR and AIMR in children of school age and to elucidate the variation in prevalence rates. Therefore the methodology of prevalence studies performed since 1960 was critically evaluated and a distinction was made between ascertained and true prevalence estimates.