The Malleability of IQ as Judged From Adoption Studies

The Malleability of IQ as Judged From Adoption Studies

Charles Locurto (1990)
College of the Holy Cross

Estimates of the malleability of IQ depend largely upon the results of adoption studies. Moderate estimates presently range between 20 to 25 points. It is here argued that the standard adoption study in fact provides little unequivocal evidence of IQ’s malleability. Further, the most prominent adoption studies of contrasted environments, studies wherein the biological family’s socioeconomic status is clearly different from that of the adoptive family, provide malleability estimates that are more modest than has been claimed elsewhere.

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Gains de QI par l’effet de la pratique : Absence du facteur g

Dans le présent article seront analysées deux études présentant des effets ‘test-retest’ sur le QI, précisément Watkins (2007) et Schellenberg (2004, 2006). Les deux études utilisent le test d’intelligence de Wechsler. Malgré la présence d’un gain de QI, la corrélation entre changement du score et la saturation en g des sous-tests du Wechsler est négative, comme l’indiquaient déjà des études précédentes. Dans la mesure où g, l’ingrédient actif des tests cognitifs (Gottfredson, 1997), est absent des gains cognitifs, on devrait en conclure que l’effet de la pratique n’influence pas l’intelligence manifeste, mais uniquement les scores observables, et non les scores latents.

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Psychometric intelligence and achievement: A cross-lagged panel analysis

Psychometric intelligence and achievement: A cross-lagged panel analysis

Marley W. Watkins, Pui-Wa Lei, Gary L. Canivez (2007)


There has been considerable debate regarding the causal precedence of intelligence and academic achievement. Some researchers view intelligence and achievement as identical constructs. Others believe that the relationship between intelligence and achievement is reciprocal. Still others assert that intelligence is causally related to achievement. The present study addressed this debate with a cross-lagged panel analysis of WISC-III and achievement test scores of 289 students assessed for special education eligibility with a test–retest interval of 2.8 years. The optimal IQ–achievement model reflected the causal precedence of IQ on achievement. That is, the paths from IQ scores at time 1 to IQ and achievement scores at time 2 were significant whereas the paths from achievement scores at time 1 to IQ scores at time 2 were not significant. Within the limits imposed by the design and sample, it appears that psychometric IQ is a causal influence on future achievement measures whereas achievement measures do not substantially influence future IQ scores.

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The causal factor underlying the correlation between psychometric g and scholastic performance

The causal factor underlying the correlation between psychometric g and scholastic performance

Dasen Luo, Lee A. Thompson, Douglas K. Detterman (2003)


Structural equation models were fitted to covariances among 9 Cognitive Abilities Test (CAT) variables, 11 Wechsler Intelligence Scale for Children—Revised (WISC-R) subtest scores, and 3 Metropolitan Achievement Test (MAT) scaled scores, administered to a sample of 532 primary school children who participated in the Western Reserve Twin Project. The models were designed to test the hypothesis that factors representing basic cognitive processes, extracted from the nine CAT variables, were the main causal determinants for the observed correlation between psychometric g and scholastic performance, which were represented, respectively, by a general factor extracted from the WISC-R and a factor from the MAT. Structural relations between the CAT factors as the primary independent variables, psychometric g as a secondary independent variable, and scholastic performance as the dependent variables were estimated, and the R² change indicating the higher-order shared variability between g and scholastic performance was evaluated. After the influence of a CAT general factor was controlled, the WISC-R general factor accounted for about 6% of the variability in the MAT scholastic factor, as opposed to as much as 30% of the zero-order variability shared by the two variables. The results were not seriously affected by the exclusion of nonchronometric measures of the cognitive tasks from the model, suggesting that individual differences in mental speed are a main causal factor underlying the observed correlation between general intelligence and scholastic performance in children between the ages of 6 and 13.

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Phenotypic and Behavioral Genetic Covariation Between Elemental Cognitive Components and Scholastic Measures

Phenotypic and Behavioral Genetic Covariation Between Elemental Cognitive Components and Scholastic Measures

Dasen Luo, Lee Anne Thompson, and Douglas K. Detterman (2003)

The study subjected nine elementary cognitive task variables from the Cognitive Assessment Tasks (CAT) and three scholastic measures from the Metropolitan Achievement Test (MAT) to phenotypic and behavioral genetic structural equation modeling based on data for 277 pairs of same sex monozygotic (MZ) and dizygotic (DZ) twins from the Western Reserve Twin Project. Phenotypic and behavioral genetic covariation between certain elemental cognitive components and scholastic performance was examined to determine (a) whether these elemental cognitive components contribute substantially to the variance of scholastic performance; (b) whether such contributions vary across different domains of school knowledge or from specific domains to a general aptitude; (c) the behavioral genetic composition of the elemental cognitive components and the scholastic variables; and (d) how the association between the cognitive components and scholastic performance is genetically and environmentally mediated. The results of the study showed that as much as 30% of the phenotypic variance of scholastic performance was accounted for by the CAT general factor, which was presumably related to mental speed. A mainly genetic covariation was found between the mental speed component and scholastic performance, although each of the two variables was strongly influenced by both heritability and common family environment. The magnitude and etiology of the covariation were largely invariant whether mental speed was related to a common scholastic aptitude or to individual achievement measures covering different knowledge domains. Taken in conjunction with previous findings that mental speed has a substantial genetic correlation with psychometric g, and psychometric g has a mostly genetic covariation with scholastic achievement, the findings of the present study seems to point to a more global picture; namely, there is a causal sequence that starts from mental speed as the explanatory factor for both psychometric g and scholastic performance, and the etiology of the causal link is chiefly genetic.

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Mental abilities and school achievement: A test of a mediation hypothesis

Mental abilities and school achievement: A test of a mediation hypothesis

Miriam Vock, Franzis Preckel, Heinz Holling (2011)


This study analyzes the interplay of four cognitive abilities – reasoning, divergent thinking, mental speed, and short-term memory – and their impact on academic achievement in school in a sample of adolescents in grades seven to 10 (N=1135). Based on information processing approaches to intelligence, we tested a mediation hypothesis, which states that the complex cognitive abilities of reasoning and divergent thinking mediate the influence of the basic cognitive abilities of mental speed and short-term memory on achievement. We administered a comprehensive test battery and analyzed the data through structural equation modeling while controlling for the cluster structure of the data. Our findings support the notion that mental speed and short-term memory, as ability factors reflecting basic cognitive processes, exert an indirect influence on academic achievement by affecting reasoning and divergent thinking (total indirect effects: β=.22 and .24, respectively). Short-term memory also directly affects achievement (β=.22).

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Learning abilities and disabilities: Generalist genes in early adolescence

Learning abilities and disabilities: Generalist genes in early adolescence

Oliver S. P. Davis, Claire M. A. Haworth, and Robert Plomin (2009)

Social, Genetic & Developmental Psychiatry Centre, Institute of Psychiatry, King’s College London, UK

Introduction. The new view of cognitive neuropsychology that considers not just case studies of rare severe disorders but also common disorders, as well as normal variation and quantitative traits, is more amenable to recent advances in molecular genetics, such as genome-wide association studies, and advances in quantitative genetics, such as multivariate genetic analysis. A surprising finding emerging from multivariate quantitative genetic studies across diverse learning abilities is that most genetic influences are shared: they are ‘‘generalist’’, rather than ‘‘specialist’’.
Methods. We exploited widespread access to inexpensive and fast Internet connections in the United Kingdom to assess over 5000 pairs of 12-year-old twins from the Twins Early Development Study (TEDS) on four distinct batteries: reading, mathematics, general cognitive ability (g) and, for the first time, language.
Results. Genetic correlations remain high among all of the measured abilities, with language as highly correlated genetically with g as reading and mathematics.
Conclusions. Despite developmental upheaval, generalist genes remain important into early adolescence, suggesting optimal strategies for molecular genetic studies seeking to identify the genes of small effect that influence learning abilities and disabilities.

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Education Is Associated With Higher Later Life IQ Scores, but Not With Faster Cognitive Processing Speed

Education Is Associated With Higher Later Life IQ Scores, but Not With Faster Cognitive Processing Speed

Stuart J. Ritchie and Timothy C. Bates
The University of Edinburgh

Geoff Der
The University of Edinburgh and University of Glasgow

John M. Starr and Ian J. Deary
The University of Edinburgh


Recent reports suggest a causal relationship between education and IQ, which has implications for cognitive development and aging – education may improve cognitive reserve. In two longitudinal cohorts, we tested the association between education and lifetime cognitive change. We then tested whether education is linked to improved scores on processing-speed variables such as reaction time, which are associated with both IQ and longevity. Controlling for childhood IQ score, we found that education was positively associated with IQ at ages 79 (Sample 1) and 70 (Sample 2), and more strongly for participants with lower initial IQ scores. Education, however, showed no significant association with processing speed, measured at ages 83 and 70. Increased education may enhance important later life cognitive capacities, but does not appear to improve more fundamental aspects of cognitive processing.

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Genetic and environmental contributions to population group differences on the Raven’s Progressive Matrices estimated from twins reared together and apart

Genetic and environmental contributions to population group differences on the Raven’s Progressive Matrices estimated from twins reared together and apart

J. Philippe Rushton, Trudy Ann Bons, Philip A. Vernon and Jelena Čvorović (2007)

We carried out two studies to test the hypothesis that genetic and environmental influences explain population group differences in general mental ability just as they do individual differences within a group. We estimated the heritability and environmentality of scores on the diagrammatic puzzles of the Raven’s Coloured and/or Standard Progressive Matrices (CPM/SPM) from two independent twin samples and correlated these estimates with group differences on the same items. In Study 1, 199 pairs of 5- to 7-year-old monozygotic (MZ) and dizygotic (DZ) twins reared together provided estimates of heritability and environmentality for 36 puzzles from the CPM. These estimates correlated with the differences between the twins and 94 Serbian Roma (both rs = 0.32; Ns = 36; ps < 0.05). In Study 2, 152 pairs of adult MZ and DZ twins reared apart provided estimates of heritability and environmentality for 58 puzzles from the SPM. These estimates correlated with the differences among 11 diverse samples including (i) the reared-apart twins, (ii) another sample of Serbian Roma, and (iii) East Asian, White, South Asian, Coloured and Black high school and university students in South Africa. In 55 comparisons, group differences were more pronounced on the more heritable and on the more environmental items (mean rs = 0.40 and 0.47, respectively; Ns = 58; ps < 0.05). After controlling for measurement reliability and variance in item pass rates, the heritabilities still correlated with the group differences, although the environmentalities did not. Puzzles found relatively difficult (or easy) by the twins were those found relatively difficult (or easy) by the others (mean r = 0.87). These results suggest that population group differences are part of the normal variation expected within a universal human cognition.

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Nature, nurture, and expertise

Nature, nurture, and expertise

Robert Plomin, Nicholas G. Shakeshaft, Andrew McMillan, Maciej Trzaskowski (2013)

Rather than investigating the extent to which training can improve performance under experimental conditions (‘what could be’), we ask about the origins of expertise as it exists in the world (‘what is’). We used the twin method to investigate the genetic and environmental origins of exceptional performance in reading, a skill that is a major focus of educational training in the early school years. Selecting reading experts as the top 5% from a sample of 10,000 12-year-old twins assessed on a battery of reading tests, three findings stand out. First, we found that genetic factors account for more than half of the difference in performance between expert and normal readers. Second, our results suggest that reading expertise is the quantitative extreme of the same genetic and environmental factors that affect reading performance for normal readers. Third, growing up in the same family and attending the same schools account for less than a fifth of the difference between expert and normal readers. We discuss implications and interpretations (‘what is inherited is DNA sequence variation’; ‘the abnormal is normal’). Finally, although there is no necessary relationship between ‘what is’ and ‘what could be’, the most far-reaching issues about the acquisition of expertise lie at the interface between them (‘the nature of nurture: from a passive model of imposed environments to an active model of shaped experience’).

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