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A prevalence survey of mental subnormality in Northern Ireland was carried out on lines similar to an earlier investigation by MacKay (1971). In a general population of one and a half million there were 6,854 ascertained subnormal individuals. The male : female ratio was 1. 14 :1. Approximately one in three subnormal people was under residential care. The prevalence of severe subnormality (IQ less than 50) in the 15-19 year old group was 4.81. The findings do not support the "steady rate" theory of severe subnormality.
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McDonald et al. (1978) studied this question.
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