Deficiency and Delinquency: An Interpretation of Mental Testing

by James Burt Miner

In the graphic presentation of the curves of development in Figures 3 and 5 the relative position at various ages has been suggested hypothetically for those of the best ability and median, or middle ability, as well as the borderline of the deficients.

It is evident that these graphs should represent equivalent ability at each stage of development measured by as objective a scale of measurement as possible. In the graphs this scale is assumed to be composed of physical units with its zero at zero ability. The deficient group is distinguished by the portion with a grated shading. The distribution curves of individual ability we have already mentioned in connection with scales of measurement. Fig. 3 is constructed on the assumption of a normal distribution of ability at each age extending to the same zero ability. Fig. 5 on the assumption of distributions of varying form.

Otis has given a very able logical analysis of certain concepts underlying the testing of mental development ([163]). His discussion differs from the present in its aim to determine the proper mental age for particular tests, a question which I have not considered. It also supplements the present discussion by showing the changing value of the same intelligence quotient with normal distributions of ability under certain assumptions as to range of ability and decrease in the annual increments of ability with age.

(a) The Significance Of Average Curves Of Development.

Some investigators are apparently inclined to question the significance of any curve of mental development on account of the very different forms of development which they have found in particular cases. A quotation from Goddard will state this problem:

“It seems to me that there is considerable evidence that there are a good many children that develop at a normal rate up to a certain age and then slow down; some slowing down gradually and others rapidly. This is possibly accounted for by accidental conditions. Dr. Healy's case of traumatic feeble-mindedness is a good illustration of this. We have quite a good many cases, not a large percentage as yet, where it is pretty clear that they have developed very nearly normally up to the age of seven, eight or nine, so that I am very skeptical as to the possibility of formulating a rule for determining the rate of development. Many cases are uniform in slowness while others vary a great deal; some slow up more rapidly than others as has already been stated....

“Morons are not usually discovered until twelve or fourteen years of age. The picture to me of the development of the feeble-minded is rather that these different types develop each in his own way very much as the physical side develops. Different families have different determiners of development. Just as it was determined before I was born that I should be five feet, ten inches tall, I developed that height and no further. In the same way, probably, that determiner carries with it the determination of the rate of development and the time. This carries with it the fact that I should have been an average boy from birth. As a matter of fact I was very much under-size until I was fifteen or sixteen years of age. Then I shot up. Other cases are over-size. It may be a false analogy, but it seems to me to illustrate the rate at which these cases develop” ([111]).

This view raises clearly the question how far the curve of average development represents a common tendency of different individuals in development. Are the individual curves of development so varied in form that an average curve does nothing but obscure their significance? The study of individual curves of growth in height and weight by Baldwin indicates that the bigger children tend to develop earlier, the smaller later ([73]). The individual curves of mental development may be analogous. If so, the average curves may not adequately represent the common tendencies of development. Nevertheless, it is to be remembered that with height and weight the average curves do retain a decided usefulness, which nobody, I suppose, would seriously question.

An analogous problem arises when we consider the question of variations in the maturity of different mental processes. Besides the question whether the average curve is useful in view of the variation among individuals in their rates of maturity for the same process, the psychologists have a still more difficult problem about curves of general ability. These curves are built by combining the results of numerous psycho-physical tests which are very different in type. We need to raise the question whether the type of process measured by memory for digits, for example, matures at the same rate as those processes measured by other memory tests: in general, how much a single test or combination of tests represents a common process. Furthermore, we need to inquire whether processes measured by memory tests mature like those measured by tests emphasizing reasoning, imagination, motor ability and other groups of activities. We thus have the problems of the different rates of maturity of the different tested processes in the same individual and of common tendencies among these specific processes.

In order more clearly to present this problem of the significance of developmental curves for different processes, I have brought together the age norms from 8 to 14 years for 40 tests as given by different investigators. No norms were included which were not based on tests of at least 25 individuals. After 14 years the data which have been collected are open to the objection that the norms for the older ages would be seriously affected by the fact that they were obtained upon children remaining in school, usually in the elementary school, i. e., upon groups, among which a large portion of those of better or of poorer ability had been eliminated. The relative position of the norms for older ages are, therefore, not comparable with those of children who are of the ages of compulsory attendance. The results published are inadequate below 8 years for most of the tests, so I have not extended the curves to earlier ages. In 14 instances the data for boys and girls were only given separately. In these I have used the norms for the boys. A prepubertal break in a combined curve may, therefore, indicate a sex difference. In most cases the norms were given for the sexes combined, and the difference is unimportant for the points considered.