A. The Border Region for the Mature.
(a) Indication from a random group.
The passing limit for adults is unquestionably much more important than that for children since any child who once passes this limit is assured, generally speaking, of social fitness so far as intellect is concerned. He has attained a position intellectually which is sufficiently good to enable him to get along without social assistance unless he is especially deficient in will. This borderline for the mature has been so thoroughly neglected that in none of the common published forms of the Binet scale, except the new Stanford Scale, is there an attempt to define it. This seems almost incredible in view of the general use of the Binet method in diagnosing feeble-mindedness. To be sure, there are discussions of this upper limit, as we shall see, but they have usually not been embodied in the actual directions accompanying the scales which get into the hands of amateurs. Most of these directions content themselves with describing borderlines for children with no caution about the final lower limit for social survival.
The borderline for the mature is the first difficulty which a court examiner will encounter when he attempts to obtain assistance from an objective system of measurement. Very little experience will convince one that it is not enough to describe the deficient ability of an adult in terms of years of retardation. It is widely agreed that at some age during adolescence practically all the mental processes are available that will be found in the mature. From that time the advance in ability is made by attaining greater skill in specific activities through training and by increasing knowledge, rather than through a native change in the form of thinking. If mental tests mainly reach capacity for thinking, as they aim to do, rather than amount of knowledge or skill in specific work, then we are conservative in using a randomly selected group at 15 years of age for approximating the borderline on the scale for the mature.
In connection with the new Stanford Scale, Terman says: “Native intelligence, in so far as it can be measured by tests now available, appears to improve but little after the age of 15 or 16 years. It follows that in calculating the I Q (intelligence quotient) of an adult subject, it will be necessary to disregard the years he has lived beyond the point where intelligence attains its final development. Although the location of this point is not exactly known, it will be sufficiently accurate for our purpose to assume its location at 16 years” (57, p. 140).
Yerkes and Bridges in connection with their Point Scale say, “it seems highly probable that the adult level is attained as early as the sixteenth year” (225, p. 64). Kuhlmann ([138]) used 15 years as the divisor in calculating the intelligence quotient of adults and Spearman thinks that the limit of native development is reached about 15 years ([184]). He says, “That mental ability reaches its full development about the period of puberty is still further evidenced by physiology. For the human brain has been shown to attain its maximum weight between the ages of 10 and 15 years” ([184]). For the last statement he quotes Vierordt. On the contrary Wallin thinks that we need more evidence for the correctness of these hypotheses before choosing a fixed age as a divisor for adults (215, p. 67).
We are not interested in determining a divisor for an adult intelligence quotient but in fixing a conservative borderline for the mature. Admitting that the mental capacity of those 15-year-olds at the lower limit may not be like adults, nevertheless adults would be more likely to be better than worse. Borderlines for the 15-year-olds, should, therefore, be safe for adults. Moreover, the lower limits with a truly random group of 15-year-olds would probably be more reliable than an assorted group of adults subjectively chosen from different walks in life and combined in an effort to represent a random mature group. The Stanford Scale utilizes such combination of selected adults. It seems, therefore, that we are justified in utilizing the lowest percentages of randomly selected 15-year-olds as a reasonable criterion for describing the limits for adult deficiency. Surely adults below this lower limit for 15-year-olds would have questionable intellectual capacity.
The borderline for the mature being the crucial feature of a developmental scale when used for detecting feeble-mindedness, it seemed imperative to us that some effort should be made to obtain records with a random group of older-age children or adults. Goddard's results with school children were not significant above eleven years of age since the personal examinations were confined to children in the sixth grade or below. The twelve year old group in the sixth grade clearly omits the best 12-year-olds, so that the percentage method would have no significance applied to his figures for children above 11 years of age. Moreover it was obvious that the group of public school children 15 years of age or older would not give a picture of the lower end of a random group since many children drop out of school at 14. On the average those that leave are undoubtedly of lower ability than those who remain.
The most valuable data on the borderline for the mature would come from mental examinations of large random groups of adults. The impossibility of gaining the consent of adults for such examinations puts this plan out of consideration. Perhaps the next best method would be to examine all the children of 15 and 16 years of age in typical communities. It happened that we could approach this result in Minneapolis since we there had an excellent school census made from house to house covering all children under 16 years of age. The Minnesota law requires school attendance until 16 years of age unless the child has graduated from the eighth grade. Under the able direction of Mr. D. H. Holbrook of the attendance department the census of children of school age had been made with unusual care. All the children living in each elementary school district in the city were listed in a card index regardless of whether they were attending public, parochial or private schools, or had been excused from attendance for disability or for any other reason. Since we only needed to be sure to examine the lowest few per cent. of the children in ability this group of 15-year-olds could be tested by examining all those children in typical school districts in the city who had not graduated from the eighth grade. A third of the 15-year-olds were still in the eighth grade or below. Neither the compulsory attendance law nor the census would have reached the 16-year-old adequately. In most states even the 15-year-olds would have been above the compulsory school age.
There were 653 children, (322 boys,) 15 years of age living in the seven typical districts which were selected objectively for study. Among these there were 196 who had not graduated from the eighth grade. All of these latter children were examined, except one who could not be tested as she was in a hospital on account of illness. Quite a number of the children were in parochial or private schools, two were followed to the state industrial school and a number were examined at home. In order to be sure that we had not missed any institutional cases in these districts the complete list of Minneapolis children at the State School for Feeble-Minded was gone through to get any of low ability who might have been missed.