“Provisionally it might be proposed to fix at 9 years the upper level of mental debility.... We have reason to think that a development equivalent to the normal average at 9 years of age is the minimum below which the individual is incapable of getting along without tutelage in the conditions of modern life. A certain number of facts suggest this view and are mutually confirmatory. Nine years is the intellectual level found in the lowest class of domestic servants, in those who are just on the border of a possible existence in economic independence; it is, on the other hand, the highest level met with in general paralytics who come under asylum care on account of their dementia; so long as a general paralytic, setting aside any question of active delirious symptoms, has not fallen below the intellectual level of 9 years, he can keep at liberty; once he has reached that level, he ceases to be able to live in society. And lastly, when we examine in our asylums cases of congenital defect, brought under care for the sole reason that their intelligence would not admit of their adapting themselves sufficiently to the complex conditions of life, we find that amongst the most highly developed the level of intelligence does not exceed that of a normal child of 9 years of age” ([182]).
In connection with their 1911 revision of the scale Binet and Simon had stated that among 20 adults in a hospital where custodial care was provided for the deficient “we found that the best endowed did not surpass the normal level of nine or ten years, and in consequence our measuring scale furnished us something by which to raise before them a barrier that they could not pass” (79, p. 267). They, however, then expressed complete reserve as to the application of this criterion to subjects in different environments on their presumption that deficiency for the laboring class is different from that for other classes in the population.
The Germans seem to have early recognized a lower borderline for the mature than we did in this country for we find Chotzen saying in 1912 that he agreed with Binet's finding that “idiots do not rise above a mental age of three, imbeciles not over seven, and debile not over ten” (89, p. 494). Stern also quotes Binet as declaring that the moron does not progress beyond the mental age of nine (188, p. 70).
The tendency of interpretation indicated by these studies is plainly to lower the borderline for passable mature intellects until it approaches the limits which the percentage definition suggests as reasonable from our available evidence. The percentage plan thus confirms the borderline that has been approached gradually by hit or miss methods. An adult testing IX is presumed deficient, while one testing X is in an uncertain zone. The numerous studies of delinquents which have regarded adults who tested XI and even XII as deficient have seriously overestimated the problem of the deficient delinquent, as we shall see in our later chapter on tested delinquents.
B. The Border Region for the Immature.
(a) For the Binet 1908 Scale.
In attempting to adapt the percentage method of description to the border region for the immature, it is essential that the tests shall have been tried out on randomly selected groups. Neither teachers nor the examiner should pick out children to be tested, if we are to know much about the region of lowest intellects. While Bobertag's method of choosing typical groups by balancing those backward in school by those advanced, is serviceable for his purpose of determining norms, the personal element of choice involved makes the results thus obtained almost useless in determining the lower limit of ability.
In regard to the diagnosis of intellectual deficiency by the Binet 1908 or 1911 scales, we know much more about the interpretation of results obtained with the 1908 scale than with the 1911 scale. The 1908 scale was therefore used for our examinations of juvenile delinquents. The best available data on which to base a description of the borderline for the immature is that collected by Goddard ([119]). He says that he “arranged to test the entire school population of one complete school system. This system includes about five thousand population within a small city and as many more outside, so that we have, city and country, a school population of about two thousand children.... In the seventh and eighth grammar grades and the high school, the children were tested in groups.” Since only the first six grades were tested individually and only these results are published in sufficient detail to be available, we shall confine this account to the school children below the seventh grade. It must be remembered that any children of the idiot class and possibly some of the low imbeciles would not be included in his figures for they would probably have been excused from school attendance. In a small rural community it is not likely that these would be numerous enough to change the rough borderline materially. We thus have a fairly random group for a small town and its environs.
Since we cannot use Goddard's results for our purpose above the sixth grade, it is plain that we would not sufficiently approach a random distribution for any age above 11 years. In Minneapolis, for example, a recent census showed 28% of the public school children 12 years of age are in the seventh grade or above, while 6% of the better eleven-year-olds would be excluded by including only those below the seventh grade. We have therefore omitted from our calculations all of Goddard's results for children above eleven years of age as too unreliable for purposes of percentage estimations. Even his eleven-year-olds may be affected.
Although it is not clear in the published reports whether the nearest or last birthday was used, Dr. Goddard has informed me that his table shows the results for ages at the last birthday. A child is regarded as six until he has reached his seventh birthday, as is customary. Throughout this book I have followed this method of using age to mean age at last birthday, or avowed age. This is in conformity with the common use of age and with general anthropometric practise. It is less confusing and less subject to mistake or errors of record. On the whole, I believe that in statistical work avowed age is preferable to nearest age.