How often do we know the causes

The various concepts of mental retardation and its causes have led to different epidemiological estimates. The population at risk consists of survivors, but some figures include also all live born. Differences in the definition and detection of cases, the classification system used, the population studied, the timing of the study in relation to measures of the general population, the resources available for the study, and sources of the data make it difficult to compare the frequencies of retardation or the frequency of various causes obtained in different studies. However, a specific cause for mental retardation can be identified for approximately 80 per cent for persons with severe mental retardation (IQ < 50) and 50 per cent of persons with mild mental retardation (IQ 50-70 or 75). The causes of the mental retardation are estimated to be prenatal in 50 to 70 per cent, perinatal in 10 to 20 and postnatal in 5 to 10 per cent of persons. ^M1. and 1!) In general, knowledge of aetiologies is more accurate for people with severe mental retardation than for those with mild mental retardation. (!2)

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