On account of the great attention that has been paid to individual differences in recent years, on account of their importance for diagnosis, for determining the causes of deficiency, and for planning for the training of deficients, we have come almost to the point where we forget the significance of the average as the most common condition with which we have to deal. The lack of complete correlation between abilities of an individual does not make us hesitate to use the concept of his average ability; it should not make us neglect or misunderstand the significance of the position of an individual testing low down on the scale. For the problem of social care the borderline position on a scale is immensely more important than higher ability. It seems advisable, therefore, to define this borderline ability with some suitable allowance for variability in mental processes. It is far safer to judge an individual's chance of survival by his average or general tested ability than by the little knowledge that is as yet available regarding special abilities.
[4]. Augusta F. Bronner. The Psychology of Special Abilities and Disabilities. Boston, 1917, pp. vii, 269.
CHAPTER IV. WHAT PERCENTAGE IS FEEBLE-MINDED
A. Kinds of Social Care Contemplated
At first it seems like a hopeless task to try to bring harmony out of the confused estimates of the proportion of the feeble-minded in modern society. Authoritative estimates by commissions or by recognized experts range from less than 0.2% to 5.0% that is, from 2 to 50 per thousand. Further study of these estimates shows that they reflect not so much a difference in expert opinion about the same problem as differences in the problems which were considered in making the estimates. As soon as we compare only those estimates that have been made to answer the question, what percentage of low grade minds should be provided with a certain form of social care? it is rather surprising how much less the discrepancy becomes. An analysis of important estimates will therefore be undertaken in order to try to discover some of the sources of disagreement.
The most significant thing about an estimate is that the estimator is thinking of providing for his group of deficients in a special way. This is the purpose of the estimates. Three important groups of the mentally deficient now demand attention. They are: (1) The group which, for moral and eugenic reasons, society is justified in isolating for life or an indefinite period. (2) The group which needs special simple industrial training in order to get along with social assistance without isolation. These deficients may be cared for in their home towns by special schools, public guardians, and after-care committees. (3) The group which needs special school assistance, but is socially passable after leaving school. These individuals are incapable of competing in school with their fellows, but they are able to get along in the simplest employments without social assistance. We may designate these three groups as those needing (1) social isolation, (2) social assistance, and (3) only school assistance. The largest estimates of feeble-mindedness, it will be found, include the third group, while the smallest intend to include only the first group. The first and second groups are clearly below the limit of feeble-mindedness designated by the verbal definition of the British Commission. They are socially unfit. The language of that definition is ambiguous enough to include the third group, but the plan of the Commission, judged by its consideration of the number to be sent to special schools, would regard only the first two classes as feeble-minded. Following this common conception I have regarded those in the third group as above the feeble-minded. It will help to find harmony among the estimates if we estimate separately those mentally deficient enough to need social isolation, social assistance, and only school assistance. This discrimination of the retarded by the kind of social care needed should also make the social definition more useful.
B. Estimates of the School Population Versus the General Population
Before we consider the percentage estimates in detail for these different forms of social care, let us note the effect on them of two other considerations. The first of these is the discrepancy between estimates of the proportion of feeble-minded among school children and estimates as to the proportion in the general population. Since feeble-mindedness is regarded as a permanent arrest of mental development occurring at an early age and usually due to hereditary causes, it is plain that a school child who is feeble-minded would be expected to remain so for life. Nevertheless we find that estimates of 0.3% of the general population are accompanied by estimates of 1.0% or 2.0% of the school population as feeble-minded. I have not been able to find any careful attempt to account for these discrepancies. The excessive mortality among the feeble-minded is hardly adequate to explain so great a difference.
It is interesting to note some of these comparisons. Goddard, for example, considers it conservative to estimate that 2% of the school population is “feeble-minded” (112, p. 6). In the same publication he says: “There are between 300,000 and 400,000 feeble-minded persons in the United States” (p. 582). Since the elementary school enrollment is about 20,000,000 ([208]), the feeble-minded school children alone on his first estimate would account for 400,000 feeble-minded in the United States without allowing for any feeble-minded outside of the ages in the elementary school.