When the problem of measuring mental capacity was first taken into the laboratory, the modern definition of a mental function began to be formulated. It became apparent that a mind must be judged by its product. The measurement of performance is the only approach there is, or probably ever will be, to the measurement of mind. On this basis it was found impossible to identify or measure any such function as “the reason,” “the memory,” “the observation,” “the imagination,” “the will,” and similar supposed entities. A mental function came to be defined as “an actually or possibly observable event in behavior.” Thus, memorizing digits, detecting absurdities, and reading English print are examples of mental functions, in the sense in which the term is used throughout the chapters of this discussion.
Other terms which are used to refer to performances or “events in behavior,” are abilities and capacities. A prolonged discussion might be conducted, in an attempt to assign different technical meanings to these words, and to bring out fine shades of distinction among them. For instance, it might be claimed that “ability” should be reserved to signify capacity plus the skill acquired by practice, if any; while “capacity” should mean the innate aptitude, apart from all training. However, since capacity in this sense can never be known, but can only be inferred from the degree of actual performance, under controlled conditions, it hardly seems necessary to maintain such distinctions for our purpose. Refinements of nomenclature will, therefore, be avoided, and the terms mental function, capacity, and ability will be used interchangeably, to denote performance which depends on the inborn integrity and sensitivity of the individual.
By way of clarifying the definition of a mental function as “an actually or possibly observable event in behavior,” we may quote from Spearman’s presentation of the distinction between “observation” as a mental function, and “observation of birds’ nests.” Spearman says: “Suppose, for instance, that a school boy has surpassed his fellows in the observation of birds’ nests. His victory has, no doubt, depended in part on his capacity for the general form of activity known as ‘observation.’ But it has also depended on his being able to apply this form of activity to the matter of birds’ nests; had the question been of tarts in the pastry cook’s window, the laurels might well have fallen to another boy. A further influence must have been exercised by the accompanying circumstances; to spy out nests as they lie concealed in the foliage is not the same thing as to make observations concerning them in the open light of a natural history museum. Again, to discover nests at leisure is different from doing so under the severe speed limits prescribed by the risk of an interrupting gamekeeper. The boy’s rank may even depend largely on the manner of estimating merit. Marks may be given either for the gross number or for the rarity of the nests observed; and he who most infallibly notes the obvious construction of the house-sparrow may not be the best at detecting the elusive hole of the kingfisher.” One cannot, therefore, identify and measure “observation.” One can only measure “observing birds’ nests, of all kinds, at leisure,” or “observing rare birds’ nests, under stress of pursuit,” and so forth, which are “actual or possible events in behavior.”
As one may glean further from Spearman’s discourse, it has been shown that most of the mental functions performed by men are not elementary, but consist of the coördination of complex factors, capable of analysis. Reading the English word “cat” from a printed page is, for instance, a very complex function.
The application of quantitative methods to the study of mental functions as thus defined, quickly revealed the fact that human beings, sampled at random, in large numbers, do not fall into distinct types. On the contrary, they yield one unbroken curve of distribution in the function measured, clustering around a single type (or mode). In all mental functions which have been measured, there has been found but one type—the average human type—from which the individual members of the species deviate in degree (though not in kind). The majority of individuals deviate but slightly from this biologically established type or mode. “The typical” in ability is, indeed, by definition, what the greatest number of people can do. From this performance of the average or typical person, a few individuals deviate widely in the direction of superiority, while a corresponding few deviate widely in the direction of inferiority. No doubt the conspicuousness, because of their infrequency, of extreme deviates in respect to any given function (or capacity) has led to the notion of separate types of mankind. Mental measurement shows clearly that men cluster closely around one type in mental traits, just as they do in such physical traits as height and weight. All men can be no more divided into the dull and the bright, than they can be divided into the tall and the short. The eye can see that most persons are best described as medium, in height.
This principle of one type, with deviations in both directions from it, in a measured trait, holds throughout organic nature. The study of it in all its bearings is called the study of individual differences. When the traits involved are mental, we speak of the psychology of individual differences. It is one of the marvelous facts about human beings that of all the millions born, no two are just equal in possession of a given trait, except by chance; and no two are identical in their combinations of traits, for the infinite possibilities of permutation practically exclude identity by chance. These combinations, which go to make up personality, are combinations of amounts of the same traits. This must be clearly understood. The mental classification of men under different “kinds” is a myth. All show the same kinds of functions; but they show all degrees of performance in these functions, within limits which are extremely wide, with multitudinous possibilities of combinations of functions, in different amounts of each.
There are, therefore, not types. There is one type—the typical or most frequently occurring amount of performance in a function—from which there is divergence among the individuals born, in various degrees. Is it possible to construct a picture of this fact, so that it may become concrete through visual representation? Psychologists have given us many such pictures, in the forms of curves platted from their measurements. We may cite as an example, Seashore’s curve of distribution for the ability to discriminate among intervals of time, which is one element in musical sensitivity. Seashore measured a large number of adults in this respect, with the result that is pictured in Figure 1.
Where the curve rises to its greatest height, at its peak, there the greatest number of those measured fall in respect to this function. That is, therefore, the human type, in sense of time. The typical individual has that amount of this trait. On each side of the type fall deviating persons, their frequency decreasing rapidly as the amount of deviation becomes greater. Very few persons in ten thousand have that amount of sensitivity to time represented by 95–100; and, on the other hand, very few are so inferior as to fall at the lowest point measurable on this scale. The typical person has that amount of the trait represented by 85–75, approximately. Distinct types, such as “sensitive” and “insensitive,” do not appear, as a result of mathematical distribution. But a few extreme deviates from the typical appear,—the superior in sensitivity and the inferior in sensitivity.
Fig. 1.—Distribution of ability to discriminate among intervals of time, the subjects being adults. (From Seashore’s The Psychology of Musical Talent. Reproduced by courtesy of Silver, Burdett and Company, and of The Columbia Graphophone Company.)