Individual Differences

Although recapitulation theory influenced Hall’s approach to child study, the direct influence of evolutionary theory on child study was slight (Charlesworth, 1992). However, the theory of evolution strongly influenced the study of individual differences. For natural selection to serve as the primary mechanism of evolution, variation in species populations was necessary for the selection of traits that were the basis for adaptation and survival within different and changing environments. Francis Galton, a cousin of Darwin, contributed to the history of psychology through his measures of physical and mental characteristics of individuals who visited his Anthropometric Laboratory. The measures of physical characteristics such as head size, arm length, height and weight, and performance characteristics such as reaction time and sensory acuity, used by Galton and adapted from the tasks of the psychological laboratories, were employed as mental tests of intelligence. Head size, for example was (falsely) assumed to indicate brain size and intellectual capacity, and speed of responses and visual acuity were assumed to indicate adaptability and survival capability. The term intelligence came to be used to designate differences among individuals in their capacity for such complex behaviors as reasoning and problem solving rather than to denote differences among species in adapting to the
environment, the more common use of the term in the nineteenth century.
James McKeen Cattell, who had studied with Hall at Johns Hopkins before earning his PhD with Wundt, pursued his interest in individual variation, labeled “ganz
Amerikanisch” by Wundt (Boring, 1950), while in Francis Galton’s London laboratory. Cattell returned to establish a laboratory at Columbia University and adapted laboratory tasks familiar to him from both Leipzig and London to identify and measure differences in reaction time, sensory sensitivity, time estimation, and memory span in undergraduate students (Sokal, 1987; Tuddenham, 1962). Like Galton, he theorized that such tasks as reaction time, sensory acuity, memory, and apprehension spans would reveal an individual’s intellectual abilities. His attempt to relate scores on
these tasks to academic performance demonstrated little relationship between the performance scores on the laboratory tests to academic performance in courses at Columbia (Sokal, 1987) but nevertheless represents an early effort to measure the intelligence of individuals. Assessing individual differences among human beings did
not necessarily result in appropriate conclusions about the consequences of evolution because of the importance of social and cultural factors in determining differences among individuals. For example, Galton’s study of sex differences in psychological characteristics reflected social and cultural views of the capabilities and proper roles for women and men rather than differences that could be attributed to evolutionary forces. This bias was common at the time and addressed by the research of one of James R. Angell’s graduate students, Helen Bradford Thompson. Her dissertation, completed at the University of Chicago in 1900 and later published
as The Mental Traits of Sex (1903), was the first systematic, experimental investigation of sex differences in motor ability, sensations, intellect, and affect. Careful, detailed analysis of the results led to her conclusion that “the
psychological differences of sex seem to be largely due, not to difference of average capacity, nor to difference in type of mental activity, but to differences in the social influences brought to bear on the developing individual from early infancy to adult years”. Hall, too, had employed evolutionary arguments to bolster stereotyped ideas about the psychological nature and proper roles of men and women. His rather unflattering assessment of women’s abilities attracted little argument from American
male psychologists of the time (see Diehl, 1986; Shields, 1975) and played a role in denying opportunities for graduate study and professional employment for women (Milar, 2000). In 1910, Helen Thompon, writing under her married name, Helen ThompsonWoolley, reviewed the literature on sex differences and asserted, “There is perhaps no field aspiring to be scientific where flagrant personal bias, logic martyred in the cause of supporting a prejudice, unfounded assertions, and even sentimental rot and drivel, have run riot to such an extent as here” (Woolley, 1910, p. 340). Similar conclusions could have been drawn about comparisons among races
begun before the development of evolutionary theory. These comparisons had also served to justify a hierarchy that placed Caucasians in a superior position, and later studies under the aegis of evolutionary theory continued to be carried out
and interpreted in terms of long-held cultural biases (see R. Guthrie, 1998).
Influenced by Cattell and Hall’s child study movement, Lightner Witmer (1867–1956), attempted to put performance on laboratory tasks to practical use in the new discipline that he labeled “Clinical Psychology” (McReynolds, 1996). The apparatus and methods of the laboratory experiment were successful in assessing differences among individuals but proved to be of little value for Witmer’s purposes (McReynolds, 1996). The failure of laboratory tasks for these applied ends led, in the case of intelligence testing, to the refinement and development of tests modeled on those of
Alfred Binet and, in Witmer’s case, to the search for more suitable methods for assisting individuals. These efforts also led to attempts to identify characteristics of individuals that, like intelligence, were both measurable and offered promise
of relevance, such as personality assessment (Allport, 1937), attitude and aptitude measures, and clinical diagnostic tests (Gregory, 1992). For many psychologists, individual differences were a distraction to the understanding of the general principles governing mind, while for others, the understanding of the individual mind was the most interesting task for psychology. The difference in emphasis and the somewhat separate paths of development of the two pursuits within psychology
came to be seen as the two disciplines of scientific psychology (Cronbach, 1957).