Prevalence of Intellectual Deficiency in New South Wales

Prevalence rates of 4 grades of mental retardation in New South Wales were calculated on the basis of data available from educational and psychiatric sources. The rates per 100,000 population were: mild retardation (I.Q. 56 to 75) 4,135; moderate retardation (I.Q. 36 to 55) 390; severe retardation (I.Q. 20 to 35) 19; and profound retardation (I.Q. 0 to 19) 14. In all grades of retardation there was a predominance of males over females, the approximate proportion being 6 to 4. An adjustment for age was made to prevalence rates of lower grades of retardation, so as to take account of their differential mortality rates. The rates of moderate, severe and profound grades together were in line with findings in the U.K. The rate of mild retardation was within 10% of the expected theoretical rate. Some of the theoretical and empirical assumptions underlying surveys of the prevalence of mental retardation are discussed critically.