Multivariate genetic analysis of cognitive abilities in an adolescent twin sample

The sources of covariation among cognitive measures of Inspection Time, Choice Reaction Time, Delayed Response Speed and Accuracy, and IQ were examined in a classical twin design that included 245 monozygotic (MZ) and 298 dizygotic (DZ) twin pairs. Results indicated that a factor model comprising additive genetic and unique environmental effects was the most parsimonious. In this model, a general genetic cognitive factor emerged with factor loadings ranging from 0.28 to 0.64. Three other genetic factors explained the remaining genetic covariation between various speed and Delayed Response measures with IQ. However, a large proportion of the genetic variation in verbal (54%) and performance (25%) IQ was unrelated to these lower order cognitive measures. The independent genetic IQ variation may reflect information processes not captured by the elementary cognitive tasks, Inspection Time and Choice Reaction Time, nor our working memory task, Delayed Response. Unique environmental effects were mostly nonoverlapping, and partly represented test measurement error. Modern theories of human cognition tend to view intelligence as a collection of varied abilities (including low, intermediate and high levels of cognitive processing), which can be organised hierarchically (Carroll, 1993; Kaufman & Lichtenberger, 1999; Roberts & Stankov, 1999). In this paper we report diverse measures of cognitive ability (information processing speed, working memory, and IQ) with the aim of understanding the genetic and environmental relationships among these correlated abilities. More specifically, we address whether the measures share the same underlying genetic factor, or whether they relate to each other through a number of different genetic factors. Findings on the significant phenotypic interrelationship among processing speed, working memory and IQ (Miller & Vernon, 1996; Salthouse, 1996) suggest that a single genetic factor will be the most predominant. Although genetic studies (Alarcon, Plomin, Fulker, Corley, & DeFries, 1998; Petrill et al., 1998; Thompson, Detterman, & Plomin, 1991) have shown that there are significant genetic correlations between perceptual speed (psychometrically derived), memory and verbal/spatial abilities (factors derived from Cognitive Abilities Test), there have been few studies that directly assess the genetic relationship among processing speed, working memory and IQ. In our previous study of the genetic relations among Choice Reaction Time (CRT; 2-, 4-, and 8-choice), Delayed Response accuracy and Full Scale IQ, two genetic factors were found to mediate the covariance among these measures (Luciano et al., 2001a). In the present paper we have added Inspection Time and Delayed Response speed measures to the previously reported battery to establish whether further separation of genetic group factors is possible with a broadened range of cognitive measures. Information processing speed as measured by elementary cognitive tasks is a conceptually distinct process from perceptual speed measured by psychometric tests. Processing speed and perceptual speed indices are correlated at only 0.16 (Kyllonen, 1993). Elementary cognitive task correlates of IQ include Correspondence: M. Luciano, Genetic Epidemiology Laboratory, Queensland Institute of Medical Research, Brisbane, Qld 4029, Australia. Tel.: + 61 7 3362 0218. Fax: + 61 7 3362 0101. E-mail: [email protected] Australian Journal of Psychology, Vol. 56, No. 2, September 2004, pp. 79 – 88. ISSN 0004-9530 print/ISSN 1742-9536 online # The Australian Psychological Society Ltd Published by Taylor & Francis Ltd DOI: 10.1080/00049530410001734874 such measures as choice reaction time, speed of scanning in short term memory, visual and auditory discrimination ability, and speed of long-term memory retrieval. In general, the RT measures demonstrate correlations of around – 0.30 with IQ, while measures without an RT component, such as perceptual discrimination speed, are higher (e.g., – 0.50) (Deary & Stough, 1996; Jensen, 1993). In this study we sample two established processing speed measures, Inspection Time and CRT. Multivariate studies of IQ or specific cognitive abilities have indicated that memory subtests (or factors) are influenced by a genetic general (g) factor, but even more so by a specific genetic factor (Cardon, Fulker, DeFries, & Plomin, 1992; Finkel, Pedersen, McGue, & McClearn, 1995; Luo, Petrill, & Thompson, 1994). There is a further report of a genetic relationship between processing speed and various short-term memory tests, including word recall, immediate and delayed text recall and figure memory (Finkel & McGue, 1993). These studies have either used short term memory tasks (which tap working memory storage) or a conglomerate measure of different memory types (e.g., shortand longterm) rather than a specific test of working memory. To measure working memory, we use a visuospatial delayed response (DR) task, which involves both short-term storage and executive function. This task requires the participant to retain information over the course of a short time delay during which the response is withheld. It is more complex than the processing speed tasks because of the requirement of inhibition, timing the motor response as well as remembering the target position while ignoring distractors. Areas of prefrontal cortex have been implicated in executive function and are activated during task performance, indicating that the DR task does tap some process in the working memory system (Geffen et al., 1997; Goldman-Rakic, 1992). While accuracy on the DR task serves as an index of working memory, the DR speed measures also have relevance to our study. Delayed response speed is measured by a response initiation time and a movement time. Initiation time refers to the speed with which a participant reacts to a cue and may be akin to simple RT; another processing speed measure. Movement time is the speed with which a person completes a response following response initiation, and it may be associated with CRT because our CRT task uses a keyboard paradigm resulting in a combined decision and movement time response. Using a classical twin design this study aimed to elucidate the proportion of variance contributed by genes and environment to the covariation among measures of Inspection Time, CRT (2-, 4-, and 8choice conditions), DR speed and accuracy responses and IQ (verbal, performance). In doing so we seek to establish the genetic and environmental factor structure that best explains the covariation among these diverse cognitive abilities.