This paper reviews the behavior-genetic literature on speed of information processing with special focus on two major issues in this field: (a) the importance of genetic and environmental influences on individual differences in performance on elementary cognitive tasks, and (b) the contributions of correlated genetic and correlated environmental effects to the relation between faster processing rates and higher intelligence scores that is consistently observed. The bulk of relevant studies points to substantial genetic influences on speed of information processing. These genetic influences, however, do not exceed the importance of genetic influences on psychometric intelligence. Moreover, small to moderate shared environmental influences on speed of information processing are usually found. The correlation between mental speed and psychometric intelligence is mainly accounted for by correlated genetic effects, but genetic effects on processing speed and intelligence are not perfectly correlated.
Key words: information processing, speed.
Tests of basic cognitive abilities or elementary cognitive tasks (ECT) have been developed to investigate the sources of individual differences in general cognitive ability or intelligence (Jensen, 1987, 1993, 1998). Such tasks are often viewed as indicators of biologically influenced cognitive processes that are involved in problem-solving ability (Petrill, Thompson, & Detterman, 1995). Note that biologically influenced does not necessarily imply genetically influenced as there may be effects of biological variables like nutrition that do not reflect the influence of genes.
To gain a better understanding of the role that genetic and environmental influences play at different levels of cognitive functioning, several issues have to be addressed, among them the extent that cognitive abilities at different levels are heritable (i.e., influenced by genes). Another important problem concerns the relation between speed of information processing and psychometric intelligence: The available evidence suggests a moderate negative correlation between reaction time (RT) in ECTs and psychometric intelligence, and thus the etiology of this relationship in terms of genetic and environmental sources should be explored. But the state of knowledge with regard to these issues varies: Whereas numerous studies investigated genetic and environmental influences on general intelligence and more specific cognitive abilities (e.g., verbal, spatial and memory; see Plomin, DeFries, McClearn, & Rutter, 1997 for a review), the available evidence on the heritability of ECTs is by far less extensive.
The first study that addressed this issue was conducted by McGue, Bouchard, Lykken, and Feuer (1984). In the subsequent decade, three studies were published that reported heritabilities of elementary cognitive tasks (Vernon, 1989; McGue & Bouchard, 1989; Boomsma & Somsen, 1991), and two additional studies explored the extent in how far the correlation between processing speed and psychometric intelligence reflected shared genetic or shared environmental factors (Ho, Baker, & Decker, 1988; Baker, Vernon, & Ho, 1991). In all these studies, the samples were rather small and different ECTs were employed, thus limiting the generalizability of the findings. Recently however, some behavior-genetic studies on elementary cognitive tasks have been published in which larger and more representative samples were investigated (Petrill et al., 1995; Petrill, Luo, Thompson, & Detterman, 1996; Rijsdijk, Vernon, & Boomsma, 1998; Neubauer, Spinath, Riemann, Borkenau, & Angleitner, in press). In this article, these studies and their findings will be reviewed, and their implications will be explored.
Early Behavior-Genetic Studies on Elementary Cognitive Tasks
McGue et al. (1984) investigated samples of 34 MZ and 13 DZ adult twin pairs made up, in part, of participants in the Minnesota Study of Twins Reared Apart. …