# Popular Science Monthly/Volume 49/October 1896/A Measure of Mental Capacity

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* A MEASURE OF MENTAL CAPACITY.
By Dr. EMIL KRAEPELIN,
PROFESSOR OF PSYCHIATRY AT HEIDELBERG.
(From an Address delivered in behalf of the Heidelberger Frauenverein.)
WE are able to calculate almost precisely the amount of work any given machine—as a steam engine or an electric-lighting plant—is capable of performing, and the amount of fuel that will be required to develop the calculated power. When we come to man we are much less certain, although a skillful army surgeon can tell almost at a glance whether the recruit standing before him is strong enough to meet the requirements of the service, and there are machines in the market that will inform us in what time we can pull a given weight to a given height. But we have no measure that we can apply to the capacity for mental work, and no units of mental valuation. The most we can do is to compare the intellectual capacity of one man with that of another by the mental results they have severally achieved in practice. When we wish to test the fitness of a candidate for a position of trust or responsibility, we subject him to an examination, which relates, however, mostly to what he has learned, and from which we guess in a rather indirect way what he may be capable of doing in the future, and with relation to other matters than those on which we examine him; and the test is very often deceptive: for those who have made the most brilliant displays in the examination frequently fail in capacity to make practical application of what they have learned only theoretically; or they fail by irregularity, frequent and marked changes in their disposition to work, want of endurance, or too great dependence on external conditions, of which the examination gives no prediction. Such efforts as have been made to obviate this difficulty have hitherto failed to meet their object.
It has, however, recently become possible to reach fairly approximate conclusions concerning mental capacity, such as ​wise even a long personal acquaintance with the candidate could not afford, and to determine with considerable exactness the working power of individuals in simple mental tasks. The measure is afforded by determining the number of small, similar problems resolved by the subject in a given time—such, for example, as numbering letters, reading, the learning by heart of series of numbers or syllables, and the continuous addition of columns of numbers. In the last-mentioned method the person under trial is set to adding figures ranged one under another in a book printed expressly for that purpose, for a considerable time, without stopping—under some circumstances, for several hours. When the sum reaches a hundred, the hundred is simply carried on and added to the excess in units. A bell sounds every five minutes, when the candidate draws a line after the last-added number. At the end of the trial it is easy to determine how many numbers the person can add every five minutes.
The candidates—all of nearly equal degrees of advancement, and of about the same age—varied greatly in the speed of their execution, the more rapid ones adding two and a half times as many numbers in five minutes as the slower ones. This proves that facility in reckoning is largely peculiar to the individual. Accuracy, however, was not considered. If that had been brought in, some of the results might have been materially different.
It further appeared that the speed of the additions increased regularly with each effort, but not equally with the different subjects, so that it was possible sometimes for a slower calculator eventually to pass ahead of the next quicker one. This improvement in facility was, however, subject to limitation, and is less in each repetition—as, for example, twenty-five per cent from the first trial to the second; fifteen per cent from the second to the third; and about six per cent from the third to the fourth—till a point is finally reached when there is no further increase. This capacity for improvement through practice appears also to be an individual quality. The permanence of the acquisitions obtained through it has not been sufficiently investigated; but they seem in the end gradually to wear out, and the rapidity of the wearing-out process to vary with the persons.
Of an opposite character to this is the far more rapidly increasing effect of fatigue, which always causes a diminution of efficiency, however much it may at first be temporarily balanced by the improvement through exercise. When it has once gained the upper hand, a speedy and unintermitted decline of efficiency ensues. The time when this shall take place depends on the degree of capacity already reached, the personal peculiarity, and casual influences.
​The differences in the susceptibility of different persons to fatigue are very interesting. Every person, as a rule, possesses a course of efficiency peculiar to him, which works itself out in the same manner during any particular period of labor. Some display in single efforts first an increase and then after some time a decrease of efficiency; they are least readily fatigued. Others, registering a depression of efficiency after the first quarter of an hour, betray a very great susceptibility to fatigue. All the transitions are observable between these two forms, but each individual generally follows the same course according to his personal peculiarity.
The susceptibility to fatigue is observed in all possible examination tasks, and may therefore be considered to represent a bottom principle of the individual personality, which, while it may be influenced within certain limits, as a general rule measurably determines the capacity of men for work.
Other means of measuring the capacity of a subject are afforded by the ease with which he is diverted from his task, or his susceptibility to disturbing influences from without and from within; his elasticity, or the readiness with which he recovers from the effects of fatigue or diversion; and the way he is affected by taking food, physical exercise, and the time he has for sleep. In each and all of these fields of inquiry the result obtained has to be complemented finally by the estimation of the qualitative value of the work accomplished.
We may infer from this passing review that it is actually possible to express important properties of mental personality in measurable, generally comparable, determinations. But we are still far from being able to apply such measurements to the purposes of daily life. Yet, while we fail if we attempt to draw certain lessons from the matured, complicated organization of the grown-up man, the simpler, still growing mental equipment of the child affords a more fruitful field for study and is more subject to our influence.
The question then presents itself for investigation of the mental endurance of our school children. The school requires its pupils to perform daily a specified amount of mental work, while it is really not clearly known whether the childish brain is actually able to fulfill the demand without suffering lasting damage. The young men I experimented upon, whose facility in addition fell off at the beginning of the second hour, had already had their working powers exercised and tested by responding to the demand of the school and of the university. Against them was a child two years old, who gave plain evidences of weariness after only a few minutes of fixed attention. Valuable researches on this subject have been made by Prof. Burgerstein, of Vienna, who ​composed four series of problems in addition and multiplication, the written solution of each of which would require at least ten minutes. He gave them, mostly during the earlier school hours, to pupils of different classes, between eleven and thirteen years of age, so that the pupils would have to make four calculations ten minutes long. Five minutes' pause was given between each problem and the next. The whole experiment thus lasted fifty-five minutes, or about the usual length of a school hour. One hundred and sixty-two pupils took part in the exercises, and the results were so nearly uniform that their trustworthiness can not be doubted. The first result was a notable increase of facility in the several sections of the experiment. The number of numbers counted up was about forty per cent larger in the last section than in the first. It was found, however, that not all the pupils shared equally in this advance, but that about forty-three per cent of them showed an evident sinking of efficiency at the end of the hour. The differences in personal susceptibility to fatigue previously observed among adults was also expressed here. This, however, is only a small part of the real results of the experiment. Prof. Burgerstein took pains to determine the number of mistakes committed by the pupils and the corrections they made, in order to estimate the value of the work accomplished in the several sections. Both appeared to increase from the very first, and much more rapidly than the speed of the work. It follows hence incontestably that the evidences of fatigue in the children under examination make themselves evident with increasing force from the second section of the experiment, and that in the majority of the children it is only outwardly concealed by the likewise increasing skill. The quantity of work rose, but its value underwent a constant depreciation. Similar results were obtained by the Russian Sikorski from dictation exercises, and by Höpfner in Berlin from dictations to boys nine years old.
The general result of these still too limited investigations of the susceptibility of school children to fatigue is the incontestable fact that the demands which the schools make upon the mental capacity of their pupils are far in excess of what they should be.
Yet this work is never continuous, but is interrupted by numerous pauses for rest, which doubtless have considerable influence on the progress of fatigue. The results of Burgerstein and Höpfner's experiments would have been much more unsatisfactory if brief pauses had not been interpolated between the different working spells. The remarkable fact was brought out in my experiments with adults in addition, in which pauses of ten minutes were interposed between the half-hour tasks, that the efficiency immediately after each pause was much higher than at any time before. This result is explained simply by the different velocities ​with which the influence of practice and that of fatigue are lost. Fatigue passes away, comparatively very quickly, while the gain from practice, as already mentioned, is plainly demonstrable after weeks and even months. Thus it happens that with intervals of days or weeks each succeeding series of experiments begins with a quickness in calculation which is much greater than the highest achievement of the former experiment. The same takes place likewise after the short breathing pauses, as long as these pauses are sufficient to overcome in a measure the fatigue that has begun.
Since through the interpolation of pauses from work the otherwise inevitably sinking efficiency is kept at a nearly even height, the length of the periods of rest ought to be so adjusted that the injurious effects of fatigue should never acquire a predominance over the facility acquired by practice. If the experiment, however, is carried too far, the short pauses will no longer counterbalance the effect of fatigue, and the capacity to work will become null. For this reason the resting spells, if they are really to accomplish their purpose, should not only be much larger than they are now in our schools, but should succeed one another at shorter intervals and should be increased as the teaching is protracted.
The picture which we have to compose on the basis of the experiments under consideration is a gloomy one. While a quarter of an hour of simple work is enough to develop the first signs of fatigue in a twelve-year-old pupil, lessons of several hours' duration, interrupted only by a few short pauses, should soon lead to complete mental exhaustion. The demand on attention is much too long, the breathing spells are much too short, for healthy efficiency to be maintained only remotely.
The picture is, however, too darkly drawn. What I have sketched could take place only if the schools attained what they are striving for with all their means. Kind Nature has provided a safety valve for the salvation of our growing youth, the value of which can not be too highly estimated—inattention. Only by effort, and only for a short time, can we force a measurable concentration of the full force of attention upon the solution of the problem; care is therefore always taken in the school that the time of the session shall not be regarded wholly as a time of work. Burgerstein, indeed, thought that through the pauses he introduced the relation between effort and relaxation might be imitated in a regulated school hour. But these experiments seem to me to show to a certainty that our children would necessarily fall into mental disorder if they were really forced to work with full attention for forty minutes in each school hour. That, in fact, only a few are seriously injured by overwork in school is due to those interruptions to study and those incidents in ​teaching that give the pupils happy opportunities to loosen the reins of their tired attention and forget the hard present. "One can compel children to sit and be still," says Burgerstein, "but he must not mistake; they will still in many cases take mental rest, or make a change for themselves, and not follow the course of the teaching if they are tired." Hence arises the unexpected consequence that, under the present extension of instruction, tedious teachers are a necessity.
To a certain extent the dangers of mental overwork have been recognized for a long time. All those efforts to introduce physical exercises into the school hours have in view, to a greater or less extent, the defense of the childish brain against the imminent dangers of a one-sided tension by alternating mental and muscular exertion. Gymnastic and movement exercises, manual training, and singing and drawing, to a certain extent, are intended to furnish rest-pauses for recovery from mental weariness and the gradual restoration of the previous efficiency. For this purpose such have been interposed at intervals to relieve the strictly mental work.
The physical exercises are doubtless of considerable value toward the complete building up of the personality, but they must be regarded as relaxations only within certain limits. It is, at any rate, fundamentally false to regard physical effort as in any way a suitable preparation for mental labor. Protracted experiments, pursued under my direction, have given the result that a simple walk of from one to two hours diminishes the mental efficiency in adults at least as much as about an hour's work in addition. The same is the case to a more limited extent with much less important bodily efforts. It is well known to pupils and teachers that the greater the interval of active play, the longer time is required for collecting the faculties before returning to mental work. From these experiments has arisen the demand that physical exercises should not be regarded in the plan of teaching as relaxations; and the demand for hard mental work should not be imposed on the pupil till after a rest from them.
By far the most important compensation for all effects of fatigue is sleep. Everybody, even the man mentally most inert, develops when awake a mass of mental effort which he can not afford continuously without suffering. We need, therefore, regularly recurring periods in which the consumption of mental force shall be slower than the continuous replacement. The lower the degree to which the activity of the brain sinks, then, the more rapid and more complete the recovery.
The mental vigor of most men is usually maintained at a certain height for the longest time in the forenoon. The evidences of fatigue come on later at this time of day than in the evening, ​when the store of force in our brain has been already considerably drawn upon by the whole day's work. If no recovery by sleep is enjoyed, or it is imperfect, the consequences will invariably make themselves evident the next day in a depression of mental vigor as well as in a rise in the personal susceptibility to fatigue. The rapidity with which one of the persons I experimented upon could perform his tasks in addition sank about a third after a night journey by railway with insufficient sleep. Another experimenter could detect the effects of keeping himself awake all night in a gradual decrease of vigor lasting through four days. This observation was all the more surprising, because the subject was not conscious of the long duration of the disturbance, and was first made aware of it incidentally by the results of continued measurements on the causes of the manifestations of fatigue.
These experiments admonish us to give special attention to the question of sleep with men who work with their minds. This is of more especial importance for the growing generation, because the susceptibility to fatigue, and consequently the need of sleep, are much greater in the youthful brain than in that of adults. The average duration of sleep has been studied by Axel Key in Swedish pupils of different ages. He found that it ranges from nine hours in children ten years old down to seven hours in pupils of eighteen years. Children ten years old were found who slept only six, and some of seventeen or eighteen years who had to satisfy themselves with four hours!—a result which is really astonishing. Axel Key is certainly right when he assumes that the mass of Swedish school children of all ages are daily deprived of one or two hours of their needed sleep, to say nothing of those unfortunate ones who can sleep only half the time or less which is required for their healthy mental and bodily development.
The amounts of sleep required by different men are very various, for they are dependent on the deepness of the slumber. There are persons who sleep so soundly that a surprisingly short time spent in sleeping is enough for them. On the other hand, we know that for many idiosyncrasies a length of sleep which is quite enough for the average of men is much too short.
Besides sleep, which limits the waste for a certain time and favors the restoration of what has been consumed, we need for the maintenance of our working strength the assimilation of food. By means of food the substances are introduced to the tissues which they require for their constant renewal. Sleep alone can indeed retard for a long time the continued destruction of the organs by the processes of life, as it does in the hibernation of animals; but there comes a point at last when only the introduction of fresh restorative matter can assure the continued ​maintenance of the body. This necessity comes more speedily upon workingmen than on men of leisure, earlier upon children than upon mature persons. An infant at the breast could be entirely deprived of food for only a short time without serious harm, while a sound man, with other conditions favorable, can bear a privation of several days.
A time of several hours passes between the taking of food and its complete utilization in the body. During this time, especially after hearty meals, the mental vigor is diminished in a marked degree. Later it gradually rises, and the susceptibility to fatigue diminishes.
When now we look back at the conditions we have discovered that control mental vigor, we conclude that our children are exposed by the extent and arrangement of study-work in the schools to great perils for their mental and physical development. The questions that press upon us on this matter are of such importance that we all have reason to give them our full, undivided attention. We are only at the beginning of a real hygiene of mental labor, but the results we have obtained in this research, fully indicating the nature and operation of the dangers, point with equal clearness to the character of the preventive and remedial measures which should be sought and applied.