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.
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.