INDIVIDUAL PSYCHOLOGY, or DIFFER ENTIAL PSYCHOLOGY. That branch of psychology which deals with the individual differences be tween mind and mind, as distinct from general psychology (see PSYCHOLOGY ) , which deals with mental constants and uniformities. It thus in cludes what Mill called 'ethology' and Bahnsen 'eliaracterology,' though it is as much wider than this discipline as the concept of mind is wider than that of character. Its first principal problem is (1) to determine the range and nature of mental variations. This falls into two part problems. The individual psychologist must (a) ascertain how and to what extent mental pro cesses vary from one individual to another, and (h) trace the relation of the processes to one an other in a single mind, thus discovering whether they are mutually independent, or whether there are certain fundamental processes upon which the rest depend. The answers to these two ques tions form the contents of differential psychology proper. We have, however, further to inquire (2) into the conditions of the variations which we have established; we must investigate the influences of heredity, climate, position in life, adaptation, education, sex, age, etc. And lastly (3) we must examine, classify, and explain the expressions of mental difference in handwriting, physiognomy, characteristic gesture, literary style, etc.
The first of the three main problems is psy chologically the most important. The observed differences between mind and mind have led, in the first place. to the elaboration of a doctrine of psycholo,gical types, which—seeing that the mind experimented on in the laboratory is always an individual mind—promises to he of value, not only for individual, but also for experimental psychology (q.v.). Practically all the more com plex mental functions (ideation, attention, mem ory, judgment, action, assimilation, constructive ability) evince typical differences. Unless we take account of them we cannot adequately inter pret our experimental results. Conversely, it is clear that the careful and exact work of the laboratory is peculiarly adapted to bring out the differences which individual psychology is seek ing. More difficult is the second part of the
problem: to determine the interrelation of the variable processes within a single mind. Here we have three methods at our disposal. The first is that of abnormal cases; we may take ad vantage of instances where there is extreme de velopment or enfeeblement (perhaps even loss) of some processes, to study the resulting modifica tion of other processes. We may ask, e.g. how loss of memory affects imagination, attention, power of observation, etc. The second is that of isolated variation; we may, by experimental means, vary a single mental process, and see if this variation involves changes in other processes in the same mind. The third is that of 'tests;' we choose in advance a number of processes for study, and proceed to examine them in a number of individuals, noting whether the individual differences in the selected processes run parallel to one another, and correspond in a regular man ner. This correspondence or the lack of it will show the degree of correlation obtaining between the mental functions studied. The method is extremely laborious, but is at present more avail able than any other, and should, in the long run, lead to valid results. Binet and Henri, e.g. sug gest the following tests: (1) Memory: Visual memory of geometrical design, memory of sen tences, musical memory, memory of colors and of figures. (2) Nature of mental images. (3) Imagination: Passive (finding forms or faces in ink-blots). constructive (development of an essay theme), of design (composition or completion of a picture), and literary (construction of sen tences from given substantives or verbs). (4) Duration and range of attention. (5) Compre hension: Observation (analysis of a machine) and discrimination (definition of synonyms). (6) Suggestibility. (7) .'Esthetic and moral sen timents. (8) Muscular force and strength of will (persistence in muscular effort). (9) Motor skill and sureness of eye.
Consult: Stern, licher Psychologic der indi viduellen Differenzen (Leipzig, 1900), which con tains a bibliography: Kraepelin. Psychologische Arbeiten (Leipzig, 1896) ; Titchener, Experi mental Psychology (New York, 1901).