Certain important correlations, furthermore, have been found between the level of intelligence and the level of character. The great in mind, it may be said briefly, are also great in spirit. "General moral defect commonly involves intellectual inferiority. Woods and Pearson find the correlation between intellect and character to be about .5.... General moral defect is due in part to a generally inferior nervous organization."[1]

[Footnote 1: Thorndike: Educational Psychology (1910), p. 224.]

One other important correlation must be noted. While gifts and capacities are specific, superiority in a given trait commonly involves superiority in most others. Exceptional talent in one direction in most cases involves exceptionality in many other respects. While talents are not indiscriminately transferable from one field to another, the same complex of traits which makes a person stand out preëminently in a given field, say law, would make him stand out in any one of half a dozen different fields into which he might have gone. There seems to be no evidence that extraordinary capacity in one direction is balanced by extraordinary incapacity and stupidity in others. The fact that individuals differ not only in specific traits but in general mental capacity has, also, certain obvious practical consequences. It means that there are present in society, in the light of recent tests in the army, an unexpectedly large number of individuals below the level of normal intelligence. One in five hundred, Thorndike estimates, is the "frequency of intellectual ability so defective as to disturb the home, resist school influence, and excite popular derision." These are clearly liabilities in the social order. On the other hand, there is a large number above the level of average intelligence. The importance of this group for human progress can hardly be overestimated. As we have seen in other connections, progress is contingent upon variation from the "normal" or the accustomed, and such variation from the normal is initiated in the majority of cases by members of this comparatively small super-normal group. If civilization is to advance it must capitalize its intelligence; that is, educate up to the highest point of native ability. But in any case, its chief guarantee of progress lies in the comparatively small group in whom native ability is exceptionally high. For it is among this group that original thinking, invention, and discovery almost exclusively occur.

Causes of individual differences. Among the chief causes of individual differences may, in general, be set down the following: (1) Sex, (2) Race, (3) Near Ancestry or Family, (4) Environment. The particular fund of human nature which an individual displays, that is, his specific native endowments, as they appear in practice, will be a resultant of these various causes. In the study of each of these characteristics, we should be able ideally to eliminate all the others and to consider them each in isolation.

The influence of sex. In the case of sex, for example, we should not confuse individual differences due to the fact of sex with individual differences due to divergent training given to each of the sexes. In scientific experiments to determine sex differences in mental traits, there have been careful attempts to eliminate everything but the factor of sex itself. Thus in Karl Pearson's studies of fifty twin brothers and sisters, the factors of ancestry and difference of training and age were practically eliminated.

In so far as allowance can be made for other contributing factors, studies of individual differences due to sex have revealed, roughly speaking, the following results. There have been, in the field of sensory discrimination and accuracy of motor response, slight—and negligible—differences of responses made by male and female. The subjects stated were, in most cases, selected so far as possible from the same social strata, social and intellectual interest, and background.[1]

[Footnote 1: As, for example, the members of the graduating and junior classes of the co-educational college at the University of Chicago, studied by Dr. Thompson.]

Thorndike reports the general results of such tests as follows:

The percentages of males reaching or exceeding the median ability of females in such traits as have been subjected to exact investigation are roughly as follows:

In speed of naming colors and sorting cards by color and discriminating colors as in a test for color blindness24
In finding and checking small visual details such as letters33
In spelling33
In school "marks" in English35
In school "marks" in foreign languages40
In memorizing for immediate recall42
In lowness of sensory thresholds43
In retentiveness47
In tests of speed and accuracy of association48
In tests of general information50
In school "marks" in mathematics50
In school "marks" (total average)50
In tests of discrimination (other than for color)51
In range of sensitivity52
In school "marks" in history55
In tests of ingenuity63
In accuracy of arm movements66
In school "marks" in physics and chemistry68
In reaction time70
In speed of finger and arm movement71