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Genetic Covariance Among Measures of Information Processing Speed, Working Memory, and IQ

Genetic Covariance Among Measures of Information Processing Speed, Working Memory, and IQ

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0001-8244/04/0100-0041/0 © 2004 Plenum Publishing Corporation 41 Behavior Genetics, Vol. 34, No. 1, January 2004 (© 2004) A Genetic Investigation of the Covariation Among Inspection Time, Choice Reaction Time, and IQ Subtest Scores Michelle Luciano, 1,3 Margaret J. Wright, 1 Gina M. Geffen, 2 Laurie B. Geffen, 2 Glen A. Smith, 1 and Nicholas G. Martin 1 Received 23 Oct. 2002—Final 27 May 2003 Information processing speed, as measured by elementary cognitive tasks, is correlated with higher order cognitive ability so that increased speed relates to improved cognitive performance. The question of whether the genetic variation in Inspection Time (IT) and Choice Reaction Time (CRT) is associated with IQ through a unitary factor was addressed in this multivariate genetic study of IT, CRT, and IQ subtest scores. The sample included 184 MZ and 206 DZ twin pairs with a mean age of 16.2 years (range 15–18 years). They were administered a visual ( -figure) IT task, a two-choice RT task, five computerized subtests of the Multidimensional Aptitude Bat- tery, and the digit symbol substitution subtest from the WAIS-R. The data supported a factor model comprising a general, three group (verbal ability, visuospatial ability, broad speediness), and specific genetic factor structure, a shared environmental factor influencing all tests but IT, plus unique environmental factors that were largely specific to individual measures. The gen- eral genetic factor displayed factor loadings ranging between 0.35 and 0.66 for the IQ subtests, with IT and CRT loadings of 0.47 and 0.24, respectively. Results indicate that a unitary fac- tor is insufficient to describe the entire relationship between cognitive speed measures and all IQ subtests, with independent genetic effects explaining further covariation between processing speed (especially CRT) and Digit Symbol. KEY WORDS: Inspection time; Choice RT; IQ subtests; processing speed; multivariate genetic modeling. INTRODUCTION Behavioral genetic studies (Luciano et al., 2001a; Posthuma et al., 2001) have recently confirmed that In- spection Time (IT), like other information processing speed measures, is genetically correlated with IQ. In these studies the relationship between IT and IQ was shown to be entirely mediated by genes. The next in- teresting question to ask is whether this genetic asso- ciation reflects a unitary speed factor influencing diverse processing speed measures and IQ or whether this genetic factor is specific to IT and IQ. The present study has collected IT and Choice Reaction Time (CRT) measures of processing speed and investigates the genetic interrelationship among these measures with IQ subtest scores. Phenotypic findings demonstrate that speed mea- sures derived from diverse elementary cognitive tasks are moderately intercorrelated and their relationship with IQ is generally invariant across task type (Barrett et al., 1982; Saccuzzo et al., 1994; Vernon, 1983). One interpretation of this finding is that the relationship be- tween different processing speed indices and IQ is due to a single factor, perhaps reflecting some aspect of central nervous system functioning (e.g., Jensen, 1993).
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