DIFFERENTIAL PSYCHOLOGY is that branch of psychology which deals with the differences found among indi viduals and groups in mental traits and performances. Individual differences are attributable broadly to the two opposed, but closely interwoven, forces of nature and nurture, or heredity and environment. In the first category are the intrinsic factors such as race, sex, age, immediate ancestry, etc. ; under the latter head are the extrinsic factors, viz.—the social, educational, cul tural, physical (e.g., disease) and other agencies which shape and mould the developing individual from birth until death.
In a general sense, the complex and more recently acquired traits tend to be more variable than the simple, biologically more fundamental ones. The feeble-minded, for example, differ least from the normal in physical and motor abilities and most in the ability to use language, deal with "ideas," and employ abstract thought. Even among groups relatively homogeneous as to general ability, we find greater variability in those tests requiring the education of intricate verbal relations, in learning and association, than in those tests designed to measure the speed of voluntary movement, rote memory, or recognition. Individuals do not fall into sharply separated groups or "types" in mental, any more than in physical, traits. People differ widely, for example, in the vivid ness and kind of mental imagery which they possess. In other traits the same condition obtains; mediocrity is the status most commonly encountered, marked superiority or inferiority being relatively and equally infrequent.
The results of many careful studies of primitive peoples point to the conclusion that these races do not differ markedly from modern Europeans in general sensory equipment, such as keenness of vision and hearing, sensitivity to pain and pressure, and delicacy of the skin senses. Simple "intelligence" tests of the "form board" variety (a form board is a board containing cut out depressions into which blocks of different shapes and sizes are to be fitted) show no large differences between the whites and many primitive folk (e.g., Eskimos, American Indians, etc.), although a few races, the Igorot, the Negrito and the Pygmies, do no better than low grade and even imbecile whites. The inferiority of the negro to the white in mental capacity has often been asserted as the result of comparative studies, but it is difficult to say how much of difference found is due to native as against cultural factors. Negro high school children in New York city remain in school longer, are older on the average, and are inferior in scholastic work to whites of approximately the same social status (Mayo). The greater the admixture of white blood, the closer does the negro approach the white in performance (Ferguson). Intelligence tests given to large groups of whites and negroes in the American army place the negro below the white both in tests of the language and non-language variety. Several investigations have shown the negro to be more overtly emotional and less inhibited in his reactions than the white (Crane). The American Indian ranks consistently below the white man on tests of mental capacity ; the greater the admixture of white blood, the smaller the deviation from the performance of the white. There is some recent evidence to indicate that the Indian, while slower in motor performances than the white, is more accurate and painstaking.
In comparative studies of general intelligence made in America, Chinese children have ranked slightly lower than whites in lan guage tests (English) but better in rote memory. Japanese chil dren do not differ significantly in intelligence from comparable groups of whites, due allowance being made for social and cul tural status. The most extensive comparative data on the intel lectual differences as among national groups is that obtained from the tests given in the American army during the World War. These tests indicated a superiority of those foreign-born men from northern countries of Europe (the "Nordics") over those from central and southern Europe (the "Alpines" and "Mediter raneans"). Selection plus differences in language, schooling, age and culture probably account for much if not all of the difference found.
Comparative studies of men and women have indicated few differences which might be attributed to the factor of sex, apart from social and cultural influences. The differences within either sex are far greater than the differences between the two sexes. Women have been reported to have a finer sense of touch, better colour discrimination, and to be faster in verbal association; men to be superior, on the average, in the discrimination of dif ferences in weight and visual magnitudes, and in the speed of motor response (reaction time). In general, females are superior in tests of memory and in foreign languages; males in tests of logical construction and in problems involving abstract and space relations (Thompson). Women are somewhat less stable emo tionally than men. They have been asserted also to be more inter ested in people, more religious, more patient and more sym pathetic, while men are more interested in athletic activity, have a superior sense of humour and are more independent (Pearson, Heymans and Wiersma). Girls are definitely better in school work than boys. This result has been explained as due to the slower physical development of boys, to the greater docility of girls and to native differences in ability.
The fact that men have excelled women in practically every field of endeavour is probably to be attributed to social, cultural and traditional factors, and to the greater physical strength and endurance of men, rather than to native differences in endowment. One explanation is based upon the reputed greater variability within the male sex. Greater variability, i.e., greater range of ability, would tend to produce more gifted men than women, even though the average man and woman ranked about the same in ability. The fact of greater variability in the male sex has been disputed, however, by competent investigators, and the ques tion is still an open one.
Investigators are fairly well agreed that there is a regular and progressive increase in ability as measured by mental tests from infancy up to and through adolescence (14-18 years). This mental growth parallels roughly the steady increase in physical size and strength. After adolescence growth in most mental traits increases slowly, if at all, while in many cases it ceases alto gether. Processes which are based upon experience, such as judg ment, "reasoning ability" in practical situations, rational learning, etc., probably improve up through middle life and show no very definite losses until old age (Thorndike). There is a general loss in the speed and flexibility of mental operations in old age. Sensory acuity in general—auditory, visual and tactual—is con siderably reduced and there is some loss in learning ability, speed of reaction and memory. Curiosity and enthusiasm are lost as physical and mental vigour wane. A part, at least, of the fall in general intelligence score with increasing age may be attributed to the loss in speed of response, and also to the removal, by a relatively long-time interval, from school practice with the kind of operations called for by the tests. Suggestibility, or the ability to resist suggestion, is another complex trait which has been found to differ with age and sex. Tests applied to children indicate that ability to "resist" suggestions increases progressively with age from 6 or 7 through adolescence (M. Otis). Girls are slightly more resistant than boys but the differences are not reliable.
The influence of immediate ancestry is best studied by corn paring those of like ancestry and vice versa. Careful studies of twins have shown that they are more alike both mentally and physically than ordinary brothers and sisters. The fact that older twins are no more alike than younger twins argues for the greater potency of heredity in shaping mental traits. Brothers, and brothers and fathers, are more nearly alike, as measured by correlation of traits (both mental and physical) than are un related individuals. Brother and sister, brother and brother, and sister and sister, are also more alike in such traits as vivacity, temper, assertiveness and other character traits than unrelated persons (Pearson). No doubt a considerable part of the mental resemblance noted among those of the same family or closely related families, such as habits of speech and thought, common opinions, ideas and attitudes, is due to a common fund of environ mental influences and associations. But over and above all of this, there is, in the opinion of most competent workers in the field, a large share attributable to native factors.
See W. Stern, Differentielle Psychologie (Leipzig, 1921) ; E. L. Thorndike, Educational Psychology, vol. iii. (New York, 1914) ; G. M. Whipple, Manual of Mental and Physical Tests, Parts I. and II. (Balti more, 1921) . (H. E. G.)