Sally J. Wadsworth

The heritability of general cognitive ability increases linearly from childhood to young adulthood

Although common sense suggests that environmental influences increasingly account for individual differences in behavior as experiences accumulate during the course of life, this hypothesis has not previously been tested, in part because of the large sample sizes needed for an adequately powered analysis. Here we show for general cognitive ability that, to the contrary, genetic influence increases with age. The heritability of general cognitive ability increases significantly and linearly from 41% in childhood (9 years) to 55% in adolescence (12 years) and to 66% in young adulthood (17 years) in a sample of 11 000 pairs of twins from four countries, a larger sample than all previous studies combined. In addition to its far-reaching implications for neuroscience and molecular genetics, this finding suggests new ways of thinking about the interface between nature and nurture during the school years. Why, despite lifes ‘slings and arrows of outrageous fortune’, do genetically driven differences increasingly account for differences in general cognitive ability? We suggest that the answer lies with genotype–environment correlation: as children grow up, they increasingly select, modify and even create their own experiences in part based on their genetic propensities.

Differential Genetic Etiology of Reading Disability as a Function of IQ

To test the hypothesis that the genetic etiology of reading disability differs as a function of IQ, composite reading performance data from 223 pairs of identical twins and 169 pairs of same-gender fraternal twins in which at least one member of each pair was classified with reading disability were subjected to multiple regression analysis (DeFries & Fulker, 1985, 1988). In the total sample, heritability of the group deficit in reading performance (h 2 g) was .58 (± .08). However, when the basic regression model was fitted separately to data from twin pairs with average Wechsler (1974, 1981) full scale IQ scores below 100 or 100 and above, resulting estimates of h 2 g were .43 and .72, respectively, a significant difference (p ≤ .03, one-tailed). The results of fitting extended regression models to reading performance and continuous IQ data provide evidence that the genetic etiology of reading disability differs as a linear function of IQ (p ≤ .007, one-tailed). These results suggest that IQ is relevant for the diagnosis of reading disability and that environmental influences may be more salient as a cause of reading difficulties in children with lower IQ scores.

Longitudinal Twin Study of Early Literacy Development: Preschool and Kindergarten Phases.

We conducted behavior–genetic analyses of kindergarten reading, spelling, phonological awareness, rapid naming, and spoken sentence processing in 172 pairs of monozygotic and 153 pairs of same-sex dizygotic twin kindergarten children sampled in the United States and Australia. We also modeled progress from preschool to kindergarten in literacy-related variables, with larger numbers of twins contributing to the preschool phase. Reading, phonological awareness, and rapid naming at kindergarten showed substantial effects of genes and modest effects of shared environment; spelling was influenced by genes and environment equally; and sentence processing was affected primarily by shared environment. Longitudinal analyses indicated that the same genes affect phonological awareness in preschool and kindergarten but that a new genetic factor comes into play in rapid naming as letters and digits are introduced in kindergarten. At preschool, print knowledge and phonological awareness share one source of genetic influence, which in turn affects reading and spelling in kindergarten. Phonological awareness is subject to a second genetic factor, but only the one it shares with print also influences kindergarten reading and spelling. In contrast to the genetic effects, a single source of shared environment affects preschool print knowledge and phonological awareness and kindergarten reading. The results are discussed in the context of theoretical and practical issues in literacy development.

Understanding the complex etiologies of developmental disorders: Behavioral and molecular genetic approaches

Objective: This article has 2 primary goals. First, a brief tutorial on behavioral and molecular genetic methods is provided for readers without extensive training in these areas. To illustrate the application of these approaches to developmental disorders, etiologically informative studies of reading disability (RD), math disability (MD), and attention-deficit hyperactivity disorder (ADHD) are then reviewed. Implications of the results for these specific disorders and for developmental disabilities as a whole are discussed, and novel directions for future research are highlighted. Method: Previous family and twin studies of RD, MD, and ADHD are reviewed systematically, and the extensive molecular genetic literatures on each disorder are summarized. To illustrate 4 novel extensions of these etiologically informative approaches, new data are presented from the Colorado Learning Disabilities Research Center, an ongoing twin study of the etiology of RD, ADHD, MD, and related disorders. Conclusions: RD, MD, and ADHD are familial and heritable, and co-occur more frequently than expected by chance. Molecular genetic studies suggest that all 3 disorders have complex etiologies, with multiple genetic and environmental risk factors each contributing to overall risk for each disorder. Neuropsychological analyses indicate that the 3 disorders are each associated with multiple neuropsychological weaknesses, and initial evidence suggests that comorbidity between the 3 disorders is due to common genetic risk factors that lead to slow processing speed.

Gender Ratios for Reading Difficulties.

The prevalence of reading difficulties is typically higher in males than females in both referred and research-identified samples, and the ratio of males to females is greater in more affected samples. To explore possible gender differences in reading performance, we analysed data from 1133 twin pairs in which at least one member of each pair had a school history of reading problems and from 684 twin pairs from a comparison sample with no reading difficulties. Although the difference between the average scores of males and females in these two samples was very small, the variance of reading performance was significantly greater for males in both groups. We suggest that a greater variance of reading performance measures in males may account at least in part for their higher prevalence of reading difficulties as well as for the higher gender ratios that are observed in more severely impaired samples.

Genetic and Environmental Influences on Vocabulary and Reading Development.

Genetic and environmental relations between vocabulary and reading skills were explored longitudinally from preschool through Grades 2 and 4. At preschool there were strong shared-environment and weak genetic influences on both vocabulary and print knowledge but substantial differences in their source. Separation of etiology for vocabulary and reading continued for word recognition and decoding through Grade 4, but genetic and environmental correlations between vocabulary and reading comprehension approached unity by Grade 4, when vocabulary and word recognition accounted for all of the genetic and shared environment influences on reading comprehension.

Cognitive ability and academic achievement in the Colorado Adoption Project: a multivariate genetic analysis of parent-offspring and sibling data

To test the hypothesis that the etiology of covariation among measures of cognitive ability and academic achievement is due at least in part to shared genetic influences, data from 198 adoptive and 220 nonadoptive families participating in the Colorado Adoption Project were subjected to multivariate behavioral genetic analyses. Data on measures of cognitive ability (verbal comprehension and perceptual organization) and academic achievement (reading recognition and mathematics achievement) from related and unrelated sibling pairs tested at age 7, as well as from adoptive and nonadoptive parents, were analyzed. Phenotypic analyses confirmed previous findings of moderate correlations among measures of cognitive ability and achievement, averaging about .35. Although 54% of the covariation between reading and mathematics achievement was due to influences shared with verbal ability, a significant proportion of this covariation was independent of the cognitive ability measures. Heritabilities for the various measures were moderate, ranging from .21 to .37. Moreover, genetic influences accounted for 33–64% of their phenotypic covariation; for example, 33–60% of the observed correlations between verbal comprehension and the achievement measures, 64% of those between perceptual organization and the achievement measures, and 63% of that between reading recognition and mathematics achievement were due to shared genetic influences. Similar to the results of the phenotypic analysis, nearly half of the genetic covariance between reading and mathematics achievement was independent of cognitive ability. Their remaining covariance was due primarily to nonshared environmental influences.

Longitudinal Stability of Reading-Related Skills and Their Prediction of Reading Development

Individual differences in word recognition, spelling, and reading comprehension for 324 children at a mean age of 16 were predicted from their reading-related skills (phoneme awareness, phonological decoding, rapid naming, and IQ) at a mean age of 10 years, after controlling the predictors for the autoregressive effects of the correlated reading skills. There were significant and longitudinally stable individual differences for all four reading-related skills that were independent from each of the reading and spelling skills. Yet the only significant longitudinal prediction of reading skills was from IQ at mean age 10 for reading comprehension at mean age 16. The extremely high longitudinal latent-trait stability correlations for individual differences in word recognition (.98) and spelling (.95) left little independent outcome variance that could be predicted by the reading-related skills. We discuss the practical and theoretical importance of these results and why they differ from studies of younger children.

Parent-offspring resemblance for reading performance at 7,12 and 16 years of age in the Colorado Adoption Project

BACKGROUND The study aimed to conduct the first analysis of CAP parent-offspring resemblance for reading performance in children aged 7, 12 and 16 years, and to assess the etiology of individual differences in reading performance of children at 16 years of age. METHOD The Reading Recognition subtest of the Peabody Individual Achievement Test was administered to children in the Colorado Adoption Project (CAP) at 7, 12 and 16 years of age, and to their adoptive and nonadoptive parents when the children were 7 years of age. RESULTS Resulting parent-offspring correlations in adoptive families were not significant at any age, but correlations between scores of nonadoptive control parents and their offspring were significant at all three ages. CONCLUSIONS Results obtained from behavioral genetic model-fitting analyses of data from parents and their children tested at age 16 are consistent with results of studies of twins and siblings indicating that individual differences in reading performance are due substantially to genetic influences. In contrast, environmental transmission from parents to offspring was negligible, suggesting that environmental influences on individual differences in the reading performance of children are largely independent of parental reading performance.

Heritability of High Reading Ability and its Interaction with Parental Education

Moderation of the level of genetic influence on children’s high reading ability by environmental influences associated with parental education was explored in two independent samples of identical and fraternal twins from the United States and Great Britain. For both samples, the heritability of high reading performance increased significantly with lower levels of parental education. Thus, resilience (high reading ability despite lower environmental support) is more strongly influenced by genotype than is high reading ability with higher environmental support. This result provides a coherent account when considered alongside results of previous research showing that heritability for low reading ability decreased with lower levels of parental education.