Let us suppose the subject being tested is 9 years of age. If he goes as far in the tests as normal 9-year-old children ordinarily go, we can say that the child has a “mental age” of 9 years, which in this case is normal (our child being 9 years of age). If he goes only as far as normal 8-year-old children ordinarily go, we say that his “mental age” is 8 years. In like manner, a mentally defective child of 9 years may have a “mental age” of only 4 years, or a young genius of 9 years may have a mental age of 12 or 13 years.
Special characteristics of the Binet-Simon method.
Psychologists had experimented with intelligence tests for at least twenty years before the Binet scale made its appearance. The question naturally suggests itself why Binet should have been successful in a field where previous efforts had been for the most part futile. The answer to this question is found in three essential differences between Binet’s method and those formerly employed.
1. The use of age standards.
Binet was the first to utilize the idea of age standards, or norms, in the measurement of intelligence. It will be understood, of course, that Binet did not set out to invent tests of 10-year intelligence, 6-year intelligence, etc. Instead, as already explained, he began with a series of tests ranging from very easy to very difficult, and by trying these tests on children of different ages and noting the percentages of successes in the various years, he was able to locate them (approximately) in the years where they belonged.
This plan has the great advantage of giving us standards which are easily grasped. To say, for illustration, that a given subject has a grade of intelligence equal to that of the average child of 8 years is a statement whose general import does not need to be explained. Previous investigators had worked with subjects the degree of whose intelligence was unknown, and with tests the difficulty of which was equally unknown. An immense amount of ingenuity was spent in devising tests which were used in such a way as to preclude any very meaningful interpretation of the responses.
The Binet method enables us to characterize the intelligence of a child in a far more definite way than had hitherto been possible. Current descriptive terms like “bright,” “moderately bright,” “dull,” “very dull,” “feeble-minded,” etc., have had no universally accepted meaning. A child who is designated by one person as “moderately bright” may be called “very bright” by another person. The degree of intelligence which one calls “moderate dullness,” another may call “extreme dullness,” etc. But every one knows what is meant by the term 8-year mentality, 4-year mentality, etc., even if he is not able to define these grades of intelligence in psychological terms; and by ascertaining experimentally what intellectual tasks children of different ages can perform, we are, of course, able to make our age standards as definite as we please.
Why should a device so simple have waited so long for a discoverer? We do not know. It is of a class with many other unaccountable mysteries in the development of scientific method. Apparently the idea of an age-grade method, as this is called, did not come to Binet himself until he had experimented with intelligence tests for some fifteen years. At least his first provisional scale, published in 1905, was not made up according to the age-grade plan. It consisted merely of 30 tests, arranged roughly in order of difficulty. Although Binet nowhere gives any account of the steps by which this crude and ungraded scale was transformed into the relatively complete age-grade scale of 1908, we can infer that the original and ingenious idea of utilizing age norms was suggested by the data collected with the 1905 scale. However the discovery was made, it ranks, perhaps, from the practical point of view, as the most important in all the history of psychology.
2. The kind of mental functions brought into play.
In the second place, the Binet tests differ from most of the earlier attempts in that they are designed to test the higher and more complex mental processes, instead of the simpler and more elementary ones. Hence they set problems for the reasoning powers and ingenuity, provoke judgments about abstract matters, etc., instead of attempting to measure sensory discrimination, mere retentiveness, rapidity of reaction, and the like. Psychologists had generally considered the higher processes too complex to be measured directly, and accordingly sought to get at them indirectly by correlating supposed intelligence with simpler processes which could readily be measured, such as reaction time, rapidity of tapping, discrimination of tones and colors, etc. While they were disputing over their contradictory findings in this line of exploration, Binet went directly to the point and succeeded where they had failed.