Individual differences in speed of mental processing and human cognitive abilities: Toward a taxonomic model

Abstract Extensive research within the field of learning and individual differences focuses upon the relationship between general intelligence and process measures derived from elementary cognitive tasks (ECTs). This emphasis has ignored data indicating that cognitive abilities are best described by three levels (or strata). It has also been suggested that mental speed is a unitary construct, although it is more likely to have a complex structure. To address shortcomings evident in this literature, a multivariate investigation ( N = 179) was conducted. Factor analysis of 25 psychometric indices gave seven factors postulated under the theory of fluid (G f ) and crystallized (G c ) intelligence. Correlations between cognitive abilities and parameters derived from 11 ECTs indicated that G f (alone) was related to processing speed. This relationship is seemingly dependent upon experimental manipulations of task complexity. Regarding the factorial structure of mental speed, the results were unequivocal: Broad second-order factors may be derived from both ECTs and psychometric tests. These constructs are independent from abilities defined by accuracy scores and collectively define a general cognitive speed factor. Implications of these findings are discussed. It would appear that mental speed is more intricate than proposed, and that cognitive complexity (reflected in stimulus-response compatibility effects) plays a crucial role in its ontogenesis. In addition, several explanatory models linking intelligence to processing speed are untenable. It is likely that the search for a basic process of intelligence by means of mental speed frameworks (alone) is misguided. Recently, within the field of individual differences, there has been “an explosion of experimental studies into the speed of mental processes” (H.J. Eysenck 1995, p. 225). Various tasks, ranging from those paradigms assessing simple, psychomotor movements and on up through to measures of complex problem solving and psychometric test performance, have been employed (Stankov & Roberts 1997). The present study was designed to explore speed of processing constructs within a structural model of human cognitive abilities. Utilizing the evidence presented in Carroll's (1993) extensive reanalysis of the main data sets collected within the psychometric discipline this century, the structural model of cognitive abilities adopted is that known as the theory of fluid and crystallized intelligence (see Horn & Noll 1994; Stankov et al. 1995). In contrast, the mental speed measures selected for investigation in this study were chosen on the basis of both experimental and psychometric findings that rely on disparate accounts (e.g., information theory). Notably, mental speed constructs are not presently encapsulated within a single unifying model. Another major aim of the present study was to redress this imbalance by establishing a rapprochement between conceptual models of mental speed and human cognitive abilities.