The measurement of intelligence

The most important effect of the revision is to reduce the mental ages secured in the lower ranges of the scale, and to raise considerably the mental ages above 10 or 11 years. This difference also obtains, though to a somewhat smaller extent, between the Stanford revision and those of Goddard and Kuhlmann.

For example, of 104 adult individuals testing by the Stanford revision between 12 and 14 years, and who were therefore somewhat above the level of feeble-mindedness as that term is usually defined, 50 per cent tested below 12 years by the Goddard revision. That the dull and border-line adults are so much more readily distinguished from the feeble-minded by the Stanford revision than by other Binet series is due as much to the addition of tests in the upper groups as to the relocation of existing tests.

On the other hand, the Stanford revision causes young subjects to test lower than any other version of the Binet scale. At 5 or 6 years the mental ages secured by the Stanford revision average from 6 to 10 months lower than other revisions yield.

Three serious consequences came from the too great ease of the original Binet scale at the lower end, and its too great difficulty at the upper end:—

1. In young subjects the higher grades of mental deficiency were overlooked, because the scale caused such subjects to test only a little below normal.

2. The proportion of feeble-mindedness among adult subjects was greatly overestimated, because subjects who were really of the 12- or 13-year mental level could only earn a mental age of about 11 years.

3. Confusion resulted in efforts to trace the mental growth of either feeble-minded or normal children. For example, by other versions of the Binet scale an average 5-year-old will show an intelligence quotient probably not far from 110 or 115; at 9, an intelligence quotient of about 100; and at 14, an intelligence quotient of about 85 or 90.

By such a scale the true border-line case would test approximately as follows:—

An extended account of the 1000 tests on which the Stanford revision is chiefly based has been presented in a separate monograph. This chapter will include only the briefest summary of some of those results of the investigation which contribute to the intelligent use of the revision.

The question as to the manner in which intelligence is distributed is one of great practical as well as theoretical importance. One of the most vital questions which can be asked by any nation of any age is the following: “How high is the average level of intelligence among our people, and how frequent are the various grades of ability above and below the average?” With the development of standardized tests we are approaching, for the first time in history, a possible answer to this question.

Three important conclusions are justified by the above facts:—

The facts presented above argue strongly for the validity of the I Q as an expression of a child’s intelligence status. This follows necessarily from the similar nature of the distributions at the various ages. The inference is that a child’s I Q, as measured by this scale, remains relatively constant. Re-tests of the same children at intervals of two to five years support the inference. Children of superior intelligence do not seem to deteriorate as they get older, nor dull children to develop average intelligence. Knowing a child’s I Q, we can predict with a fair degree of accuracy the course of his later development.

The mental age of a subject is meaningless if considered apart from chronological age. It is only the ratio of retardation or acceleration to chronological age (that is, the I Q) which has significance.

The question as to the relative intelligence of the sexes is one of perennial interest and great social importance. The ancient hypothesis, the one which dates from the time when only men concerned themselves with scientific hypotheses, took for granted the superiority of the male. With the development of individual psychology, however, it was soon found that as far as the evidence of mental tests can be trusted the average intelligence of women and girls is as high as that of men and boys.

The school work of 504 children was graded by the teachers on a scale of five grades: very inferior, inferior, average, superior, and very superior. When this grouping was compared with that made on the basis of I Q, fairly close agreement was found. However, in about one case out of ten there was rather serious disagreement; a child, for example, would be rated as doing average school work when his I Q would place him in the very inferior intelligence group.

When the data were examined, it was found that practically every child whose grade failed to correspond fairly closely with his mental age was either exceptionally bright or exceptionally dull. Those who tested between 96 and 105 I Q were never seriously misplaced in school. The very dull children, however, were usually located from one to three grades above where they belonged by mental age, and the duller the child the more serious, as a rule, was the misplacement. On the other hand, the very bright children were nearly always located from one to three grades below where they belonged by mental age, and the brighter the child the more serious the school’s mistake. The child of 10-year mental age in the second grade, for example, is almost certain to be about 7 or 8 years old; the child of 10-year intelligence in the sixth grade is almost certain to be 13 to 15 years of age.

All this is due to one fact, and one alone: the school tends to promote children by age rather than ability. The bright children are held back, while the dull children are promoted beyond their mental ability. The retardation problem is exactly the reverse of what we have thought it to be. It is the bright children who are retarded, and the dull children who are accelerated.

By the Pearson formula the correlation found between the I Q’s and the teachers’ rankings on a scale of five was .48. This is about what others have found, and is both high enough and low enough to be significant. That it is moderately high in so far corroborates the tests. That it is not higher means that either the teachers or the tests have made a good many mistakes.

The tendency of teachers is to estimate a child’s intelligence according to the quality of his school work in the grade where he happens to be located. This results in overestimating the intelligence of older, retarded children, and underestimating the intelligence of the younger, advanced children. The disagreements between the tests and the teachers’ estimates are thus found, when analyzed, to confirm the validity of the test method rather than to bring it under suspicion.

Among the somewhat less satisfactory tests are the following: repeating digits (direct order), naming coins, distinguishing forenoon and afternoon, defining in terms of use, drawing designs from memory, and æsthetic comparison. Binet’s “line suggestion” test correlated so little with intelligence that it had to be thrown out. The same was also true of two of the new tests which we had added to the series for try-out.

Tests showing a medium correlation with the scale as a whole include arranging weights, executing three commissions, naming colors, giving number of fingers, describing pictures, naming the months, making change, giving superior definitions, finding similarities, reading for memories, reversing hands of clock, defining abstract words, problems of fact, bow-knot, induction test, and comprehension questions.

A test which makes a good showing on this criterion of agreement with the scale as a whole becomes immune to theoretical criticisms. Whatever it appears to be from mere inspection, it is a real measure of intelligence. Henceforth it stands or falls with the scale as a whole.

The reader will understand, of course, that no single test used alone will determine accurately the general level of intelligence. A great many tests are required; and for two reasons: (1) because intelligence has many aspects; and (2) in order to overcome the accidental influences of training or environment. If many tests are used no one of them need show more than a moderately high correlation with the scale as a whole. As stated by Binet, “Let the tests be rough, if there are only enough of them.”

Before we can interpret the results of an examination it is necessary to know how frequently an I Q of the size found occurs among unselected children. Our tests of 1000 unselected children enable us to answer this question with some degree of definiteness. A study of these 1000 I Q’s shows the following significant facts:—

Or, to put some of the above facts in another form:—

Conversely, we may say regarding the subnormals that:—

What do the above I Q’s imply in such terms as feeble-mindedness, border-line intelligence, dullness, normality, superior intelligence genius, etc.? When we use these terms two facts must be borne in mind: (1) That the boundary lines between such groups are absolutely arbitrary, a matter of definition only; and (2) that the individuals comprising one of the groups do not make up a homogeneous type.

Nevertheless, since terms like the above are convenient and will probably continue to be used, it is desirable to give them as much definiteness as possible. On the basis of the tests we have made, including many cases of all grades of intelligence, the following suggestions are offered for the classification of intelligence quotients:—

Of the feeble-minded, those between 50 and 70 I Q include most of the morons (high, middle, and low), those between 20 or 25 and 50 are ordinarily to be classed as imbeciles, and those below 20 or 25 as idiots. According to this classification the adult idiot would range up to about 3-year intelligence as the limit, the adult imbecile would have a mental level between 3 and 7 years, and the adult moron would range from about 7-year to 11-year intelligence.

There are innumerable grades of mental deficiency ranging from somewhat below average intelligence to profound idiocy. In the literal sense every individual below the average is more or less mentally weak or feeble. Only a relatively small proportion of these, however, are technically known as feeble-minded. It is therefore necessary to set forth the criterion as to what constitutes feeble-mindedness in the commonly accepted sense of that word.

The definition in most general use is the one framed by the Royal College of Physicians and Surgeons of London, and adopted by the English Royal Commission on Mental Deficiency. It is substantially as follows:—

A feeble-minded person is one who is incapable, because of mental defect existing from birth or from an early age, (a) of competing on equal terms with his normal fellows; or (b) of managing himself or his affairs with ordinary prudence.

In the second place, the criterion set up by the definition is not very definite because of the vague meaning of the expression “ordinary prudence.” Even the expression “competing on equal terms” cannot be taken literally, else it would include also those who are merely dull. It is the second part of the definition that more nearly expresses the popular criterion, for as long as an individual manages his affairs in such a way as to be self-supporting, and in such a way as to avoid becoming a nuisance or burden to his fellowmen, he escapes the institutions for defectives and may pass for normal.

The most serious defect of the definition comes from the lax interpretation of the term “ordinary prudence,” etc. The popular standard is so low that hundreds of thousands of high grade defectives escape identification as such. Moreover, there are many grades of severity in social and industrial competition. For example, most of the members of such families as the Jukes, the Nams, the Hill Folk, and the Kallikaks are able to pass as normal in their own crude environment, but when compelled to compete with average American stock their deficiency becomes evident. It is therefore necessary to supplement the social criterion with a more strictly psychological one.

For this purpose there is nothing else as significant as the I Q. All who test below 70 I Q by the Stanford revision of the Binet-Simon scale should be considered feeble-minded, and it is an open question whether it would not be justifiable to consider 75 I Q as the lower limit of “normal” intelligence. Certainly a large proportion falling between 70 and 75 can hardly be classed as other than feeble-minded, even according to the social criterion.

Examples of feeble-minded school children

M. S. Girl, age 7-6; mental age 4-6; I Q 60. Father a gardener, home conditions and medical attention fair. Has twice attempted first grade, but without learning to read more than a few words. In each case teacher requested parents to withdraw her. “Takes” things. Is considered “foolish” by the other children. Will probably never develop beyond a mental level of 8 years.

S. M. Girl, age 19-2; mental age 10; I Q approximately 65 (not counting age beyond 16). From very superior family. Has attended public and private schools twelve years and has been promoted to seventh grade, where she cannot do the work. Appears docile and childlike, but is subject to spells of disobedience and stubbornness. Did not walk until 4 years old. Plays with young children. Susceptible to attention from men and has to be constantly guarded. Writing excellent, knows the number combinations, but missed all the absurdities and has the vocabulary of an average 10-year-old. The type from which prostitutes often come.

G. V. Boy, age 10; mental age 6-4; I Q 65. Father Spanish, mother English. Family poor but fairly respectable. Brothers and sisters all retarded. In high first grade. Work all very poor except writing, drawing, and hand work, in all of which he excels. Is quiet and inactive, lacks self-confidence, and plays little. Mentally slow, inert, “thick,” and inattentive. Health fair.

Three years later G. V. was in the low third grade and still doing extremely poor work in everything except manual training, drawing, and writing. Is not likely ever to go beyond the fourth or fifth grade however long he remains in school.

A. C. Boy, age 12; mental age 8-5; I Q 70. From Portuguese family of ten children. Has a feeble-minded brother. Parents in comfortable circumstances and respectable. A. C. has attended school regularly since he was 6 years old. Trying unsuccessfully to do the work of the fourth grade. Reads poorly in the third reader. Hesitates, repeats, miscalls words, and never gets the thought. Writes about like a first-grade pupil. Cannot solve such simple problems as “How many marbles can you buy for ten cents if one marble costs five cents?” even when he has marbles and money in his hands. Described by teacher as “mentally slow and inert, inattentive, easily distracted, memory poor, ideas vague and often absurd, does not appreciate stories, slow at comprehending commands.” Is also described as “unruly, boisterous, disobedient, stubborn, and lacking sense of propriety. Tattles.”

Three years later, at age of 15, was in a special class and was little if any improved. He had, however, learned the mechanics of reading and had mastered the number combinations. Deficiencies described as “of wide range.” Conduct, however, had improved. Was “working hard to get on.”

A. C. must be considered definitely feeble-minded.

I. S. Boy, age 9-6; mental age 7; I Q 75. German parentage. Started to school at 6. Now in low second grade and unable to do the work. Health good. Inattentive, mentally slow and inert, easily distracted, speech is monotone. Equally poor in reading, writing, and numbers. I. S. is described as quiet, sullen, indifferent, lazy, and stubborn. Plays little.

Three years later had advanced from low second to low fourth grade, but was as poor as ever in his school work. “Miscalls the simplest words.” Moral traits unsatisfactory. May reach sixth or seventh grade if he remains in school long enough.

I. S. learned to walk at 2 years and to talk at 3.

In at least three of the above cases (S. M., I. S., and I. M.) the teachers had not identified the backwardness as feeble-mindedness. Not far from 2 children out of 100, or 2 out of 1000, in the average public school are as defective as some of those just described. Teachers get so accustomed to seeing a few of them in every group of 200 or 300 pupils that they are likely to regard them as merely dull,—“dreadfully dull,” of course,—but not defective.

Children like these, for their own good and that of other pupils, should be kept out of the regular classes. They will rarely be equal to the work of the fifth grade, however long they attend school. They will make a little progress in a well-managed special class, but with the approach of adolescence, at latest, the State should take them into custodial care for its own protection.

The border-line cases are those which fall near the boundary generally recognized as such and the higher group usually classed as normal but dull. They are the doubtful cases, the ones we are always trying (rarely with success) to restore to normality.

F. H. Boy, age 16-6; mental age 11-5; I Q approximately 72 (disregarding age above 16 years). Tested for three successive years without change of more than four points in I Q. Father a laborer, dull, subject to fits of rage, and beats the boy. Mother not far from border-line. F. H. has always had the best of school advantages and has been promoted to the seventh grade. Is really about equal to fifth-grade work. Fairly rapid and accurate in number combinations, but cannot solve arithmetical problems which require any reasoning. Reads with reasonable fluency, but with little understanding. Appears exceedingly good-natured, but was once suspended from school for hurling bricks at a fellow pupil. Played a “joke” on another pupil by fastening a dangerous, sharp-pointed, steel paper-file in the pupil’s seat for him to sit down on. He is cruel, stubborn, and plays truant, but is fairly industrious when he gets a job as errand or delivery boy. Discharged once for taking money.

F. H. is generally called “queer,” but is not ordinarily thought of as feeble-minded. His deficiency is real, however, and it is altogether doubtful whether he will be able to make a living and to keep out of trouble, though he is now (at age 20) employed as messenger boy for the Western Union at $30 per month. This is considerably less than pick-and-shovel men get in the community where he lives. Delinquents and criminals often belong to this level of intelligence.

What shall we say of cases like the last two which test at high-grade moronity or at border-line, but are well enough endowed in moral and personal traits to pass as normal in an uncomplicated social environment? According to the classical definition of feeble-mindedness such individuals cannot be considered defectives. Hardly any one would think of them as institutional cases. Among laboring men and servant girls there are thousands like them. They are the world’s “hewers of wood and drawers of water.” And yet, as far as intelligence is concerned, the tests have told the truth. These boys are uneducable beyond the merest rudiments of training. No amount of school instruction will ever make them intelligent voters or capable citizens in the true sense of the word. Judged psychologically they cannot be considered normal.

Children of this group should be segregated in special classes and be given instruction which is concrete and practical. They cannot master abstractions, but they can often be made efficient workers, able to look out for themselves. There is no possibility at present of convincing society that they should not be allowed to reproduce, although from a eugenic point of view they constitute a grave problem because of their unusually prolific breeding.