ABSTRACT
Because of the difficulty in making a definite diagnosis of mental subnormality early in a child’s life in all but a small number of cases, such as Down’s Syndrome (mongolism), it is not possible to state accurately its incidence (i.e. the number of new cases occurring in any population in any period, such as a year). For similar reasons, neither is it possible at a comparable age to make an accurate assessment of the prevalence (i.e. the total number of cases present in any population at any time) of mental subnormality. However, it has been generally accepted that most cases of severe subnormality should have come to light by the ages of 15–19 and that their total number then closely approximates to the true prevalence of that condition. Three recent surveys in England suggest that the prevalence rate of severe subnormality is about 4 per 1,000 (see Table II).
