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The most natural question and one that is frequently asked is:

“What, precisely, do psychological tests measure?”

It is a question that is easier to ask than to answer.

It is simple enough to say that mental tests are designed to measure the natural or inherent mental capacity of the individual, but in order to approach a clear understanding of just what this means we must first define what is meant by the term “mental capacity.”

As a matter of scientific fact, the term “mental capacity” can hardly be regarded as accurate, although it is the best term we have to describe the qualities which determine the individual’s ability to perform acts requiring conscious thought. Psychological and biological science no longer regard the human mind as something different from or in any way apart from the human body. The idea that there is such an entity as a mind that operates even in the slightest degree without reference to and independent of the physical body must be dismissed, if we are to grasp clearly the principles and methods of mental tests.

To the psychologist the mind is merely a specialized organ of the physical body. The intangible something, which is what is usually meant when persons speak of the human mind, is merely the sum of all the sensations, feelings, and judgments resulting from the delicate adjustment of an almost infinite number of nerve fibres which in themselves are a part of the physical body. One may have at birth a plentiful supply or a poor supply of potential nerve endings which are ready to be 


organized and coördinated by experience and training, but unless one has the opportunity to learn from study and experience, the desirable connections may never be developed.

The maximum capacity of the mind in any particular field is, therefore, practically determined by physical inheritance of an abundant supply of nerve endings. Thus, it may be that one individual is born with two or three times as many nerve terminals connecting at the point at the back of the eyeball where the optic nerve—which is simply a bundle or rope of nerve fibres—is attached to the mechanical apparatus upon which the reflection of objects passing before the field of vision is registered. Such an individual’s powers of observation are normally greater than those of the person of less fortunate heredity in this respect, whose lesser number of terminals of the optic nerve fibres limit his powers of optical perception and observation. Thus, one person may see at a glance a hundred details, all of which register sharply upon his consciousness, while another sees only the gross outlines and shadows, and in between is the average person who sees some details but not all.

It is well known to psychologists and biologists, although not generally understood by those who have not made a special study of these sciences, that mental capacity does not change or increase materially after the individual has reached maturity. It may be diminished through accident or disease, but the chief increase in adult life is in the volume and variety of stored-up impressions. The average girl of eighteen or boy of twenty has reached the approximate limit of his or her mental capacity. The mental tank will never grow much larger. It may be half empty or almost entirely vacant, but unless at the average age of university sophomores scientific mental tests prove the individual to be possessed of average or better than average mental capacity, it is futile to expect any great intellectual development to take place in later life.

But while the maximum capacity of the mind depends upon physical inheritance, the actual ability which is necessarily 


reflected in the scores made by a person subjected to mental tests is determined by the number and variety of nerve connections that have actually been made by environment or training. Inheritance sets the maximum limit, but as a matter of practice this maximum is never reached, or at least is so seldom reached by any individual that it can hardly be said of any human being that he has developed his mind in any direction to the utmost limit of its capacity. What we actually measure in scientific mental tests is a complex of natural or inherent abilities plus the results of education and training; because, while it is possible to a considerable extent to eliminate by properly devised tests a record of the individual’s acquired knowledge, it is practically impossible to distinguish between acquired and inherent mental ability.

Note carefully the distinction between mental ability and mental capacity. Mental ability in any individual is always less than his mental capacity. If, therefore, the mental ability as determined by scientific tests reaches the highest point on the scale of measurement, whatever that may be, it follows that the mental capacity of the individual making a perfect score is even greater than the scale is designed to measure, and how much greater can only be determined by setting up new tests based upon higher standards.

The result of any scientific test simply indicates the wealth of nerve connections that are ready to be made when the stimulus necessary to their establishment is applied. It must be understood that no one having a sound claim to the possession of scientific knowledge can contend that there are tests in existence that actually measure with complete precision the inherited as distinguished from the acquired mental characteristics. It is not conceded, however, that such precise measurements cannot be made if at any time it becomes necessary or desirable to do so. For all practical present-day purposes it is sufficient that psychological tests shall measure mental qualities which are manifested by the individual’s ability to express them by action or speech. 


The classification of individuals relative to one another and with reference to the possession of a particular mental ability or group of abilities is, therefore, necessarily based upon their relative ability to express in some intelligible and unmistakable fashion their mental power and qualities.

Back of this power of expression may lie hidden and undreamed-of capacities of which the individual himself may be vaguely conscious but of which he can give no outward manifestation. It may be, for example, that an individual is gifted with unusual powers of perception through the eyes, ears, and the senses of touch, smell, and taste but that he is deficient in nerve fibres and connections controlling the voluntary muscles by which human beings translate sensations into action and speech. This is hardly likely, as a physiological fact, to occur; the individual born with rich nerve endings in one part of the physical body is more likely to have a proportionate supply of nerve endings in all other parts of the body than to be deficient in one part and amply supplied in another. As rare exceptions, however, there are individuals who in infancy have, through accident or disease, lost certain groups of nerve connections while retaining unusually rich groups in other parts of the body. There is, of course, the most famous case in modern history, that of Helen Keller, whose auditory and optical nerve connections were lost through disease in early infancy, but whose unusual inherent mental capacity has been able to demonstrate itself through other and extraordinary means as a result of training and education.

But in ordinary life, if a man or a woman has some mental quality which does not express itself in an action which other persons can see or hear and know about, then it is not socially important. It is of consequence only to the individual and it is of little social service to undertake to measure these obscure and unexpressed and inexpressible capacities, as they can never, until they find means of expression, affect the individual’s ability or efficiency in any occupation. It is not that these things cannot 


be measured. The case of Helen Keller is one demonstration that they can be measured. Anything whatever that makes a difference in the way different individuals act is conceivably measurable, although it may not at the present time be capable of exact calculation because it has not been worth anybody’s time and effort to undertake to measure it.

To repeat, and possibly to make the preceding paragraphs more clear, let us recapitulate the different mental qualities to which reference has been made.

First, mental capacity. This is what the individual has inherited. It is the size of the tank into which sensations, perceptions, all that makes up the sum of knowledge, are poured throughout his life, by his education and his experience. While this capacity in the case of any individual can doubtless be measured, it is not necessary to measure it precisely but merely to determine whether it is large enough for the purposes in view.

Second, mental ability. This is the sum of experience and education within the limits of the individual’s mental capacity. It is represented by the individual’s ability to express himself in speech or action in the performance of any one of a number of specific acts. This mental ability can be quite definitely measured, and the possession of a certain degree of mental ability demonstrates the possession of a mental capacity greater than the ability which the individual has already reached.

Third, acquired knowledge. It is not the purpose of tests of mental capacity to measure acquired knowledge, although for many purposes it is desirable to measure the individual’s acquired knowledge in addition to his inherent ability, and in a still larger number of instances the most practical way of arriving at a fairly accurate estimate of an individual’s ability involves, among other tests, an examination into the extent of the knowledge which he has acquired through observation or training along lines definitely related to his particular occupation or pursuit in life.

The ordinary and standardized school and university examinations, 


civil-service examinations, etc., which have long been the accepted test of the individual’s ability, do not, and do not purport to, measure anything more than this last item, that of acquired knowledge. But while certain gross dimensions of individual capacity may be roughly estimated from the results of a written or an oral examination based entirely upon the subject’s stored-up knowledge, it is a matter of common knowledge, and almost every reader will be able to furnish examples out of his own experience, that such tests are frequently totally misleading. Professor Terman has reported on a comparison of the results of civil-service examinations for policemen and firemen in a California city with scientific tests applied to the individuals who successfully passed the civil-service examinations. The results were in many instances astounding. Men of such low mental capacity that they might almost be classed as feeble-minded were found to have passed with a fair degree of satisfaction the simple knowledge and physical tests set up by the city and to have obtained appointments to these responsible posts as guardians of the city’s property and lives.

While it is, therefore, the object of scientific mental tests to exclude as far as possible the acquired abilities resulting from education and environment and the knowledge that has been stored up through observation and training, it is found in practice that for all ordinary purposes it is sufficient to measure a complex of native and acquired abilities. The purpose of these tests is, in short, to discover what the individual is actually able to do, regardless of the source of that ability, provided, however, that the test of ability is so devised as to make a clear distinction between mere feats of memory and the actual exercise of original thought.

Now, it must be obvious that for the measurement of anything so complex and multi-dimensioned as the human mind, no single test or scale can be established. One cannot measure the power of visual perception, for example, by the same scale that is used to measure attentiveness or initiative. As a matter 


of fact, psychologists no longer attempt to classify human abilities as narrowly as was once the popular practice. It is almost impossible for even an expert psychologist to be sure he knows just what qualities and all the qualities any particular test measures. This is because modern psychologists no longer group reactions into general functions such as memory, attention, reason, etc., but simply describe accurately the stimulus given and the conditions under which it was given and then describe just as accurately what the reaction is. The test may be built up, for example, to measure ability to recognize and classify words, but it will also depend upon ability to read the directions, ability to attend closely to horizontal and vertical lines and upon many other correlated abilities. Any test may measure primarily a particular mental dimension or ability but it is quite certain that the resulting score will be influenced by numberless other factors than the one that the examiner is most interested in measuring.

But since one of the very best tests of intelligence is, of course, the degree to which one is able to profit by social contacts and the breadth and variety of the individual’s stored-up impressions, these extraneous or collateral qualities, which every test also more or less successfully measures in addition to the particular quality or mental dimension under direct examination, furnish useful data in arriving at a conclusion which is, after all, the main purpose sought, as to the individual’s actual abilities and potential powers.

In order, however, to get at a really useful record of the mental capacity of an individual, we must apply a variety of tests and out of the sum total of the results of these tests we are able much more accurately to gauge the degree of possession of the qualities for which we are seeking than could possibly be done by any single test, no matter how skilfully constructed. Here again science confronts the popular human demand for a panacea. But just as in medicine only the quack offers a cure-all, so, in other fields, science has no single standard to offer by which all 


results in a given field may be accomplished, and psychology cannot now or at any time in the future pretend that by a single method or a single measurement mental capacity can be gauged.

To come back to an analogy used in a previous chapter, you cannot measure all the qualities of an automobile with a ten-foot rod. Your ten-foot rod will tell you whether the wheel base is 120 inches or more or less than that. It will not tell you how much above or below 120 inches. If it be necessary for you to know that, you must provide yourself with a longer or more minutely graded measuring implement; but because the ten-foot rod does not at a glance disclose to you all that you wish to know about a particular automobile, you do not, therefore, either discredit the ten-foot rod as a measuring implement or declare that the automobile cannot be measured except by the unaided human eye.

The limitations of the ten-foot rod are perfectly obvious to you; and so, too, are the complexities of the automobile, which require a variety of instruments and tests for their proper gauging and measurement. So before you undertake to form a judgment as to the ability of a particular automobile, you either measure it yourself or, as a matter of practice, you have it measured for you by a competent engineer. You do not necessarily inquire, if you have confidence in the engineer, as to precisely what dimensions and what materials he found in every part of the car, but you respect his conclusions, knowing that they are based upon the most precise and accurate measurements possible with the aid of such instruments as science has been able to devise, and you are satisfied that the conclusions form an accurate estimate of the machine’s qualities.

The engineer who sets out to measure an automobile in all of its capacities and powers must provide himself with tachometers for measuring the engine’s revolutions, dynamometers for testing its tractive force, micrometer calipers for gauging the bore and the stroke, thermometers for measuring its temperature, galvanometers for testing its magneto and battery, and hundreds of 


other instruments, the readings of which must be assembled and studied by means of complex, comparative mathematical formulas before he can tell you what a particular automobile will do.

The human mind, it must be apparent to every reader, is not less complex than the automobile. On the contrary, it is infinitely more complex and subject to an infinitely wider range of variations. As has been pointed out above, it is not necessary for practical, every-day purposes to measure every possible variation and every one of the infinite number of dimensions of any human mind in order to ascertain the individual’s ability to succeed in the ordinary pursuits of life. But even in our ordinary, every-day affairs and contacts, in the simplest forms of employment, there are called into play such a number of different sorts of ability and mental power that there must be applied, if one is really to know of what a particular individual is capable, a large variety of tests of different kinds for measuring different powers. And for the mental measurement of individuals whose work calls for the highest development and capacity, a still larger variety of tests must be applied.

It is not always possible—in fact, it is extremely difficult—to devise tests that do not to some degree measure the mental content resulting from education and experience, in the effort to measure the mental capacity which limits and controls one’s education and experience. The qualities that determine capacity are inherent in the individual. One is born with them or is not born with them. In their whole infinite variety they are not all possessed by any one individual, and the particular grouping of mental qualities which any one person inherits is probably not possessed by any other person living or who has ever lived. Yet while individuals differ so completely that it can truthfully be said that Nature never cast two persons in the same mold, yet there are qualities possessed by all intelligent persons, the simpler and more elemental expressions of which are absolutely essential to intelligent life and existence, and these can be so 


grouped, classified, measured, and standardized as to provide a scale whereby the inherent capacity with respect to these important and essential qualities may be determined equally in the case of the totally illiterate, untrained labourer or artisan and the highly trained, educated product of a university postgraduate course.

As a matter of practical, every-day common sense, one does not expect to find, nor does one find, except as a rare exception, an individual engaged in menial or purely physical labour who is endowed with inherent mental capacity comparable to that of the university graduate. A person possessing such capacities moves out from the ranks of labour in spite of educational handicaps; the history of American business and industry is full of the romantic stories of men who have achieved success as organizers and administrators, though in many cases absolutely illiterate. Properly applied psychological tests would pass over all or nearly all of the acquired knowledge of such individuals about their particular business and related matters, and neglect also, the bulk, at least, of the acquired knowledge of the university man, and so compare merely what might be called two naked brains, the native intelligence of each being the only thing to be measured. As has been pointed out, it is difficult or almost impossible to devise tests that entirely strip the layers of acquired knowledge from the raw mental powers beneath them, but for the practical purposes of the application of psychology and psychological tests in the affairs of every-day life, this can be done within a reasonable percentage of error.

February 10
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