Our own results with a mixed group of normal, superior, dull and feeble-minded children agree fully with the above findings. In this case the two tests were separated by an interval of two to four years, and the correlation between their results was practically perfect. The average difference between the I Q obtained in the second test and that obtained in the first was only 4 per cent, and the greatest difference found was only 8 per cent.[40]
The repetition of the test at shorter intervals will perhaps affect the result somewhat more, but the influence is much less than one might expect. The writer has tested, at intervals of only a few days to a few weeks, 14 backward children of 12 to 18 years, and 8 normal children of 5 to 13 years. The backward children showed an average improvement in the second test of about two months in mental age, the normal children an average improvement of little more than three months. No child varied in the second test more than half a year from the mental age first secured. On the whole, normal children profit more from the experience of a previous test than do the backward and feeble-minded.
Berry tested 45 normal children and 50 defectives with the Binet 1908 and 1911 scales at brief intervals. The author does not state which scale was applied first, but the mental ages secured by the two scales were practically the same when allowance was made for the slightly greater difficulty of the 1911 series of tests.[41]
We may conclude, therefore, that while it would probably be desirable to have one or more additional scales for alternative use in testing the same children at very brief intervals, the same scale may be used for repeated tests at intervals of a year or more with little danger of serious inaccuracy. Moreover, results like those set forth above are important evidence as to the validity of the test method.
Influence of social and educational advantages.
The criticism has often been made that the responses to many of the tests are so much subject to the influence of school and home environment as seriously to invalidate the scale as a whole. Some of the tests most often named in this connection are the following: Giving age and sex; naming common objects, colors, and coins; giving the value of stamps; giving date; naming the months of the year and the days of the week; distinguishing forenoon and afternoon; counting; making change; reading for memories; naming sixty words; giving definitions; finding rhymes; and constructing a sentence containing three given words.
It has in fact been found wherever comparisons have been made that children of superior social status yield a higher average mental age than children of the laboring classes. The results of Decroly and Degand and of Meumann, Stern, and Binet himself may be referred to in this connection. In the case of the Stanford investigation, also, it was found that when the unselected school children were grouped in three classes according to social status (superior, average, and inferior), the average I Q for the superior social group was 107, and that of the inferior social group 93. This is equivalent to a difference of one year in mental age with 7-year-olds, and to a difference of two years with 14-year-olds.
However, the common opinion that the child from a cultured home does better in tests solely by reason of his superior home advantages is an entirely gratuitous assumption. Practically all of the investigations which have been made of the influence of nature and nurture on mental performance agree in attributing far more to original endowment than to environments. Common observation would itself suggest that the social class to which the family belongs depends less on chance than on the parents’ native qualities of intellect and character.
The results of five separate and distinct lines of inquiry based on the Stanford data agree in supporting the conclusion that the children of successful and cultured parents test higher than children from wretched and ignorant homes for the simple reason that their heredity is better. The results of this investigation are set forth in full elsewhere.[42]
It would, of course, be going too far to deny all possibility of environmental conditions affecting the result of an intelligence test. Certainly no one would expect that a child reared in a cage and denied all intercourse with other human beings could by any system of mental measurement test up to the level of normal children. There is, however, no reason to believe that ordinary differences in social environment (apart from heredity), differences such as those obtaining among unselected children attending approximately the same general type of school in a civilized community, affects to any great extent the validity of the scale.