The term 'mental subnormality' describes an incomplete or insufficient general development of the mental capacities. The term 'mental deficiency', more commonly used to describe the disability of persons suffering from incomplete development of the intelligence, is also often applied to conditions in which only the individual's emotional development is incomplete while intellectual development continues up to normal levels. These latter are also sometimes referred to in certain countries as 'moral defectives', especially in connection with certification or commitment procedures. Their disabilities, however, differ in kind from those of persons described as mentally subnormal; and it is probably preferable to deal with those who have normal intellectual abilities but incomplete or distorted emotional development which leads them into conflict with the law as 'anti-social persons with psychopathic personalities', rather than to equate them with the mentally subnormal.
The picture is further complicated by the fact that in the USA the term 'feeble-mindedness' is now coming to be replaced by the terms 'mental deficiency' or ' 'mental retardation'. In British usage, however the term 'retardation' has a developmental implication.
Although it is not possible to draw a sharp distinction between mental subnormality of varying degrees of severity, in many ways useful practice is to divide the condition into three grades: 'mild', 'moderate' and 'severe' degrees of subnormality.
Few attempts have been made to assess the prevalence of mental subnormality in different countries, and it is not possible to give definitive figures since prevalence rates depend on many factors which differ both with society and with social and economic conditions. The proportion of children regarded as educationally subnormal in different countries varies greatly according to the criteria employed. Dutch estimates based on eight large cities give a mean rate of 2.6%; French estimates range from 1.5% to 8.6%, depending on age; English educational practice aims to make provision for 1% of schoolchildren in special schools, while a further 8% or 9% are considered to require special educational provision within the ordinary school system. Varying estimates have been given in different states of the USA and in Switzerland. For adults the prevalence rates are lower, and the recognition and even the manifestation of mild subnormality in adulthood is dependent mainly on thresholds of community tolerance and the complexity of social life, both of which fluctuate widely.
Estimates are thus valid only for the time and place at which they are made. It is, however, agreed by all that the number of mildly subnormal far exceeds that of more severe cases; and English statistics which have been widely quoted suggest that among every 100 mentally subnormal persons, the following proportions will be found: 75 mild, 20 moderate, and 5 severe cases. In other words, the very great majority of the subnormal are of mild grade and potentially capable of being taught to make a fairly adequate social adaptation in appropriate circumstances.