How well does IQ predict neuropsychological test performance in normal adults? CATHERINE M. DIAZ-ASPER, 1 DAVID J. SCHRETLEN, 1,2 and GODFREY D. PEARLSON 1,3,4 1 Department of Psychiatry and Behavioral Sciences, Johns Hopkins University School of Medicine, Baltimore, Maryland 2 Department of Radiology and Radiological Sciences, Johns Hopkins University School of Medicine, Baltimore, Maryland 3 Department of Psychiatry, Yale University School of Medicine, New Haven, Connecticut 4 Olin Neuropsychiatric Research Institute, Institute of Living, Hartford, Connecticut (Received April 22, 2002; Revised April 25, 2003; Accepted May 5, 2003) Abstract The strength and nature of the association between IQ and performance on other cognitive tests has both practical and conceptual significance for clinical neuropsychology. In this study, 28 measures derived from 16 cognitive tests were analyzed as a function of IQ in 221 adults. Participants were grouped by their IQ scores as having below average (BA), average (A), or above average (AA) intelligence. Planned comparisons revealed that A adults performed significantly better than BA adults on 25 of the 28 cognitive measures, and that AA adults performed significantly better than A adults on 19 of 28 measures. Effect sizes averaged .74 for BA–A comparisons and .41 for A–AA comparisons. Linear, quadratic, and cubic functions described the relationships between IQ and cognitive test performance equally well for most individual test measures and for a composite index of test performance, whereas quadratic and cubic functions explained the proportion of abnormal performances better than a linear function. These findings confirm that IQ predicts concurrent neuropsychological performance across the entire spectrum of intelligence, but more so among persons of average IQ or less than among those with above average IQ. ( JINS, 2004, 10, 82–90.) Keywords: Intelligence, Neuropsychology, Normal adults INTRODUCTION Clinicians often use a person’s intelligence to estimate ex- pected performance on other neuropsychological tests. This is based on the assumption that performance on the latter correlates with intelligence. Discrepancies between IQ and other neuropsychological test performances contribute to clinical inferences regarding the presence of cognitive def- icits. However, the assumption of a linear relationship be- tween IQ and neuropsychological test performance has been questioned (e.g., Bell & Roper, 1998; Dodrill, 1997, 1999; Horton, 1999; Larrabee, 2000; Tremont et al., 1998). Do- drill (1997, 1999), in particular, contends that this assump- tion is a “myth” of neuropsychology. He argued that the relationship between IQ and neuropsychological test per- formance is fairly robust among persons of below-average IQ, but diminishes substantially among persons whose IQ is average or higher. In support of this argument, he exam- ined the relationship between WAIS–R (Wechsler, 1981) IQ and Halstead Reitan Battery (HRB; Halstead, 1947) test scores among 181 community-dwelling adults, and found a strong relationship between the HRB Impairment Index (HII) and intelligence at lower IQ levels, but little relationship once IQ exceeded 90 or 95 (Dodrill, 1997). Later analyses of 120 of these subjects, stratified by IQ, revealed that per- sons of average IQ outperformed those with below-average IQ on 19 of 23 HRB test variables, whereas persons with above average IQ outperformed those with average IQ on only 7 of 23 test variables. IQ correlated significantly with 10 neuropsychological tests in the below-average IQ group, no tests in the average IQ group, and 2 tests in the above- average IQ group (Dodrill, 1999). However, the findings have been challenged on a number of grounds, including the nature of his sample, which consisted almost exclu- Reprint requests to: David J. Schretlen, Ph.D., Johns Hopkins Hospi- tal, 600 North Wolfe Street, Meyer 218, Baltimore MD 21287-7218. E-mail: dschret@jhmi.edu Journal of the International Neuropsychological Society (2004), 10, 82–90. Copyright © 2004 INS. Published by Cambridge University Press. Printed in the USA. DOI: 10.10170S1355617704101100 82