sum of D[squared] = 24

n[squared] - 1 = 48

6 x sum of D[squared] = 144

6 x sum of D[squared] / n ( n[squared] - 1 )

= 1 - 144/(7 x 48)

= +.57

In order to get a full and true measure of the correlation between two tests, the following precautions are necessary:

(1) The _same individuals_ must be given both tests.

(2) The number of individuals tested must be as great as 15 or 20, preferably more.

(3) The individuals should be a fair sample of the population in regard to the abilities tested; they should not be so selected as to represent only a small part of the total range of ability.

(4) The tests should be thorough enough to determine each individual's rank in each test, with a high degree of certainty.

Sloppy testing gives a correlation nearer zero than it should be, because it "pies" the true orders to some extent.

[End footnote]

{285}

General Factors in Intelligence

If now we try to a.n.a.lyze intelligence and see in what it consists, we can best proceed by reviewing the intelligence tests, and asking how it is that an individual succeeds in them. Pa.s.sing the tests is a very specific instance of {286} intelligent behavior, and an a.n.a.lysis of the content of the tests should throw some light on the nature of intelligence.

The first thing that strikes the eye in looking over the tests is that they call for so many different reactions. They call on you to name objects, to copy a square, to tell whether a given statement is true or false, to tell wherein two objects are alike or different. The first impression, then, is that intelligence consists simply in doing a miscellaneous lot of things and doing them right.

But can we not state in more general terms how the individual who scores high in the tests differs from one who scores low? If you survey the test questions carefully, you begin to see that the person who pa.s.ses them must possess certain general characteristics, and that lack of these characteristics will lead to a low score. We may speak of these characteristics as "general factors" in intelligent behavior.

First, the tests evidently require the use of past experience. They call, not for instinctive reactions, but for previously learned reactions. Though the Binet tests attempt to steer clear of specific school knowledge, they do depend upon knowledge and skill picked up by the child in the course of his ordinary experience. They depend on the ability to learn and remember. One general factor in intelligence is therefore _retentiveness_.

But the tests do not usually call for simple memory of something previously learned. Rather, what has been previously learned must be applied, in the test, to a more or less novel problem. The subject is asked to do something a little different from anything he has previously done, but similar enough so that he can make use of what he has learned. He has to _see the point_ of the problem now set him, and to _adapt_ what he has learned to this novel situation. Perhaps "seeing the point" and "adapting oneself to {287} a novel situation"

are to be held apart as two separate general factors in intelligence, but on the whole it seems possible to include both under the general head, _responsiveness to relationships_, and to set up this characteristic as a second general factor in intelligence.

In the form board and picture completion tests, this responsiveness to relationships comes out clearly. To succeed in the form board, the subject must respond to the likeness of shape between the blocks and their corresponding holes. In picture completion, he must see what addition stands in the most significant relationship to the total picture situation. In telling how certain things are alike or different, he obviously responds to relationships; and so also in distinguishing between good and poor reasons for a certain fact. This element of response to relationships occurs again and again in the tests, though perhaps not in the simplest, such as naming familiar objects.

Besides these two intellectual factors in intelligent behavior, there are certain moral or impulsive factors. One is _persistence_, which is probably the same thing as the mastery or self-a.s.sertive instinct. The individual who gives up easily, or succ.u.mbs easily to distraction or timidity, is at a disadvantage in the tests or in any situation calling for intelligent behavior.

But, as we said before, in discussing the instincts, excessive stubbornness is a handicap in meeting a novel situation, which often cannot be mastered by the first mode of response that one makes to it.

Some giving up, some _submissiveness_ in detail along with persistence in the main effort, is needed. The too stubborn young child may waste a lot of time trying with all his might to force the square block into the round hole, and so make a poorer score in the test, than if he had given up his first line of attack and tried something else.

Intelligent behavior must perforce {288} often have something of the character of "trial and error", and trial and error requires both persistence in the main enterprise and a giving up here in order to try again there.

Finally, the instinct of _curiosity_ or exploration is evidently a factor in intelligence. The individual who is stimulated by novel things to explore and manipulate them will ama.s.s knowledge and skill that can later be utilized in the tests, or in intelligent behavior generally.

Special Apt.i.tudes

We distinguish between the general factors in intelligence, just mentioned, and special apt.i.tudes for dealing with colors, forms, numbers, weights etc. A special apt.i.tude is a specific responsiveness to a certain kind of stimulus or object. The special apt.i.tudes are factors in intelligent behavior--as we may judge from the content of the intelligence tests--only, the tests are so contrived as not to depend too much on any one or any few of the special apt.i.tudes.

Arithmetical problems alone would not make a fair test for intelligence, since they would lay undue stress on the special apt.i.tude for number; but it is fair enough to include them along with color naming, weight judging, form copying, and word remembering, and so to give many special apt.i.tudes a chance to figure in the final score.

There are tests in existence for some special apt.i.tudes: tests for color sense and color matching, for musical ability, for ability in drawing, etc.; but as yet we have no satisfactory list of the special apt.i.tudes. They come to light when we compare one individual with another, or one species with another. Thus, while man is far superior to the dog in dealing with colors, the dog is superior in dealing with odors. Man has more apt.i.tude for form, but some animals are fully his equal in sense of location and ability to find {289} their way. Man is far superior in dealing with numbers and also with tools and mechanical things. He is superior in speech, in sense of rhythm, in sense of humor, in sense of pathos. Individual human beings also differ markedly in each of these respects. They differ in these special directions as well as in the "general factors" of intelligence.

Heredity of Intelligence and of Special Apt.i.tudes

Let us now return to the question raised at the very outset of the chapter, whether or not intelligence is a native trait. We then said that the differing intelligence of different species of animals must be laid to their native const.i.tutions, but left the question open whether the differing intelligence of human individuals was a matter of heredity or of environment.

Intelligence is of course quite different from instinct, in that it does not consist in ready-made native reactions. The intelligence of an individual at any age depends on what he has learned previously.

But the factors in intelligent behavior--retentiveness, responsiveness to relationships, persistence, etc.--may very well be native traits.

But what _evidence_ is there that the individual's degree of intelligence is a native characteristic, like his height or color of hair? The evidence is pretty convincing to most psychologists.

First, we have the fact that an individual's degree of intelligence is an inherent characteristic, in the sense that it remains with him from childhood to old age. Bright child, bright adult; dull child, dull adult. That is the rule, and the exceptions are not numerous enough to shake it. Many a dull child of well-to-do parents, in spite of great pains taken with his education, is unable to escape from his inherent limitations. The intelligence quotient remains fairly {290} constant for the same child as he grows up, and stands for an inherent characteristic of the individual, namely, the rate at which he acquires knowledge and skill. Give two children the same environment, physical and social, and you will see one child progress faster than the other. Thus, among children who grow up in the same community, playing together and going to the same schools, the more rapid mental advance of some than of others is due to differences in native const.i.tution, and the IQ gives a measure of the native const.i.tution in this respect. There are exceptions, to be sure, depending on physical handicaps such as deafness or disease, or on very bad treatment at home, but in general the IQ can be accepted as representing a fact of native const.i.tution.

Another line of evidence for the importance of native const.i.tution in determining degrees of intelligence comes from the study of mental resemblance among members of the same family. Brothers or sisters test more alike than children taken at random from a community, and twins test more alike than ordinary brothers and sisters. Now, as the physical resemblance of brothers or sisters, and specially of twins, is accepted as due to native const.i.tution, we must logically draw the same conclusion from their mental resemblance.

The way feeble-mindedness runs in families is a case in point. Though, in exceptional instances, mental defect arises from brain injury at the time of birth, or from disease (such as cerebrospinal meningitis) during early childhood, in general it cannot be traced to such accidents, but is inherent in the individual. Usually mental defect or some similar condition can be found elsewhere in the family of the mentally defective child; it is in the family stock. When both parents are of normal intelligence and come from families with no mental abnormality in any ancestral line, it is practically unknown that they should have a feeble-minded {291} child; but if mental deficiency has occurred in some of the ancestral lines, an occasional feeble-minded child may be born even of parents who are themselves both normal. If one parent is normal and the other feeble-minded, some of the children are likely to be normal and others feeble-minded; but if both parents are feeble-minded, it is said that all the children are sure to be feeble-minded or at least dull.

These facts regarding the occurrence of feeble-mindedness cannot be accounted for by environmental influences, especially the fact that some children of the same family may be definitely feeble-minded and others normal. We must remember that children of the same parents need not have precisely similar native const.i.tutions; they are not always alike in physical traits such as hair color or eye color that are certainly determined by native const.i.tution.

The special apt.i.tudes also run in families. You find musical families where most of the children take readily to music, and other families where the children respond scarcely at all to music, though their general intelligence is good enough. You find a special liking and gift for mathematics cropping out here and there in different generations of the same family. No less significant is the fact that children of the same family show ineradicable differences from one another in such abilities. In one family were two brothers, the older of whom showed much musical ability and came early to be an organist and composer of church music; while the younger, possessing considerable ability in scholarship and literature, was never able to learn to sing or tell one tune from another. Being a clergyman, he desired very much to be able to lead in singing, but he simply could not learn. Such obstinate differences, persisting in spite of the same home environment, must depend on native const.i.tution.

Native const.i.tution determines mental ability in two respects. It fixes certain limits which the individual cannot {292} pa.s.s, no matter how good his environment, and no matter how hard he trains himself; and, on the positive side, it makes the individual responsive to certain stimuli, and so gives him a start towards the development of intelligence and of special apt.i.tudes.

Intelligence and the Brain

There is certainly some connection between the brain and intelligent behavior. While the spinal cord and brain stem vary according to the size of the body, and the cerebellum with the motility of the species of animal, the size of the cerebrum varies more or less closely with the intelligence of the species. It does vary also with bodily size, as ill.u.s.trated by the whale and elephant, which have the largest cerebrum of all animals, including man. But the monkey, which shows more intelligence than most animals, has also a very large cerebrum for his size of body; and the chimpanzee and gorilla, considerably surpa.s.sing the ordinary monkeys in intelligence, have also a much larger cerebrum. The cerebrum of man, in proportion to the size of his body, far surpa.s.ses that of the chimpanzee or gorilla.

The cerebrum varies considerably in size from one human individual to another. In some adults it is twice as large as in others, and the question arises whether greater intelligence goes with a larger brain.

Now, it appears that an extremely small cerebrum spells idiocy; not all idiots have small brains, but all men with extremely small brains are idiots. The brain weight of quite a number of highly gifted men has been measured in post-mortem examination, and many of these gifted men have had a very large cerebrum. On the whole, the gifted individual seems to have a large brain, but there are exceptions, and the relationship between brain size and intelligence cannot be very close. Other factors must enter, one factor being undoubtedly the fineness {293} of the internal structure of the cortex. Brain function depends on dendrites and end-brushes, forming synapses in the cortex, and such minute structures make little impression on the total brain weight.

While intelligence is related to the cerebrum as a whole, rather than to any particular "intelligence center", there is some likelihood that the special apt.i.tudes are related to special parts of the cortex, though it must be admitted that few apt.i.tudes have as yet been localized. The pretended localizations of phrenology are all wrong.

But we do know that each sense has its special cortical area, and that adjacent to these sensory areas are portions of the cortex intimately concerned in response to different cla.s.ses of complex stimuli. Near the auditory center the cortex is concerned in recognizing spoken words, and in following music; near the visual center it is concerned in recognizing printed words, in recognizing seen objects, in finding one's way by the sense of sight, etc. These special apt.i.tudes thus have a fairly definite cortical localization, and possibly others have also.

Examined microscopically, the cortex shows differences of structure in different parts, and to the structural differences probably correspond differences of function. Now it is practically impossible that such a function as attention or memory should have any localized cortical center, for these are general functions. The instincts are specialized enough to have local centers, but none have so far been localized.

What has been localized is of the nature of special apt.i.tudes.

{294}

EXERCISES

1. Outline the chapter.

2. Pick out the true statements from the following list: