Word reading and reading comprehension: stability, overlap and independence

Longitudinal twin data were analyzed to investigate the etiology of the stability of genetic and environmental influences on word reading and reading comprehension, as well as the stability of those influences on their relationship. Participating twin pairs were initially tested at a mean age of 10.3 years, and retested approximately five years later. Both word reading and comprehension were found to be highly stable, and genetic influences were primarily responsible for that stability. In contrast to studies with younger participants, no unique genetic influences were observed at follow-up testing in this older sample. High genetic correlations were obtained between word reading and reading comprehension at both ages, indicating common genetic influences. However, significant genetic influence on comprehension was also observed, independent of that on word reading. Although the phenotypic relation between the two measures appeared to decline across time, the genetic etiology of this relation was highly stable.     

Genetic and environmental influences on reading and listening comprehension

We report preliminary behaviour genetic analyses of reading and listening comprehension from The Colorado Learning Disabilities Research Center. Although the twin sample with these new measures is still of limited size, we find substantial, and significant, genetic influences on individual differences in both reading and listening comprehension. In addition, word recognition and listening comprehension each accounted for significant independent genetic influences on reading comprehension. Together, they accounted for all the genetic influence on reading comprehension, indicating a largely genetic basis for the ‘simple model’ of individual differences in reading comprehension proposed by Hoover and Gough (1990) .     

From learning to read to reading to learn: substantial and stable genetic influence

Little is known about the underlying causes and developmental patterns of stability and change in early reading abilities. In a longitudinal study of twins (n=4,291 pairs), individual differences in reading achievement assessed by teachers using U.K. National Curriculum (NC) criteria showed substantial heritabilities at ages 7, 9, and 10 years (.57-.67) and modest shared environmental influences (.10-.17). Stability in NC scores was primarily mediated genetically. There was also evidence for age-specific genetic influences at each age. Genetic influences on reading are substantial and stable during the elementary school years despite the shift from "learning to read" to "reading to learn."     

Colorado longitudinal twin study of reading disability

The primary objectives of the present study are to introduce the Colorado Longitudinal Twin Study of Reading Disability, the first longitudinal twin study in which subjects have been specifically selected for having a history of reading difficulties, and to present some initial assessments of the stability of reading performance and cognitive abilities in this sample. Preliminary examination of the test scores of 124 twins with a history of reading difficulties and 154 twins with no history of reading difficulties indicates that over the 5- to 6-year interval between assessments, cognitive and reading performance are highly stable. As a group, those subjects with a history of reading difficulties had substantial deficits relative to control subjects on all measures at initial assessment, and significant deficits remained at follow-up. The stability noted for all cognitive and achievement measures was highest for a composite measure of reading, whose average stability correlation across groups was 0.80. Results of preliminary behavior genetic analyses for this measure indicated that shared genetic influences accounted for 86% and 49% of the phenotypic correlations between the two assessments for twin pairs with and without reading difficulties, respectively. In addition, genetic correlations reached unity for both groups, suggesting that the same genetic influences are manifested at both time points.     

Gender differences in cognitive abilities of opposite-sex and same-sex twin pairs with reading disabilty

In order to compare the pattern of gender differences for cognitive measures in opposite-sex twin pairs to that in independent samples of twins from same-sex pairs, psychometric test data were obtained from four research-identified samples of children: (1) 96 pairs of opposite-sex fraternal twins in which at least one member of each pair is reading disabled; (2) 62 pairs of opposite-sex fraternal twins with no history of reading problems; (3) 167 males and 155 females from same-sex identical and same-sex fraternal twin pairs in which at least one member of each pair is reading disabled; and (4) a comparison sample of 126 males and 132 females from same-sex twin pairs with no history of reading problems. Results of multivariate analyses indicate that gender differences for cognitive measures are similar in twin pairs with and without reading disabilities. Moreover, a highly similar pattern of gender differences occurs for opposite-sex twin pairs who shared both prenatal and early postnatal influences and for independent samples of children from different families.     

Genetic and environmental etiologies of reading disability: A twin study

Previous twin studies of reading disability employed a comparison of concordance rates in identical and fraternal twin pairs as a test for genetic etiology. Recently, a statistically more powerful multiple regression analysis of twin data has been formulated to assess the importance of genetic factors in the development of reading difficulties. Application of this analysis to twin data from the Colorado Reading Project yields definitive evidence for a genetic etiology. Results from this study suggest that approximately 40 percent of the deficit observed in the disabled readers is due to genetic factors, 35 percent is due to environmental influences shared by members of twin pairs, and about 25 percent is the result of environmental factors unique to the individual and/or error variance. This work was supported in part by a program project grant from the NICHD (HD-11681).     

Predicting individual differences in reading comprehension: a twin study

We examined the Simple View of reading from a behavioral genetic perspective. Two aspects of word decoding (phonological decoding and word recognition), two aspects of oral language skill (listening comprehension and vocabulary), and reading comprehension were assessed in a twin sample at age 9. Using latent factor models, we found that overlap among phonological decoding, word recognition, listening comprehension, vocabulary, and reading comprehension was primarily due to genetic influences. Shared environmental influences accounted for associations among word recognition, listening comprehension, vocabulary, and reading comprehension. Independent of phonological decoding and word recognition, there was a separate genetic link between listening comprehension, vocabulary, and reading comprehension and a specific shared environmental link between vocabulary and reading comprehension. There were no residual genetic or environmental influences on reading comprehension. The findings provide evidence for a genetic basis to the “Simple View” of reading.     

A Twin Study into the Genetic and Environmental Influences on Academic Performance in Science in nine‐year‐old Boys and Girls

We investigated for the first time the genetic and environmental aetiology behind scientific achievement in primary school children, with a special focus on possible aetiological differences for boys and girls. For a representative community sample of 2,602 twin pairs assessed at age nine years, scientific achievement in school was rated by teachers based on National Curriculum criteria in three domains: Scientific Enquiry, Life Processes, and Physical Processes. Results indicate that genetic influences account for over 60% of the variance in scientific achievement, with environmental influences accounting for the remaining variance. Environmental influences were mainly of the non‐shared variety, suggesting that children from the same family experience school environments differently. An analysis of sex differences considering differences in means, variances, and aetiology of individual differences found only differences in variance between the sexes, with boys showing greater variance in performance than girls.     

Modeling the Etiology of Individual Differences in Early Reading Development: Evidence for Strong Genetic Influences

We explored the etiology of individual differences in reading development from post-kindergarten to post–4th grade by analyzing data from 487 twin pairs tested in Colorado. Data from three reading measures and one spelling measure were fit to biometric latent growth curve models, allowing us to extend previous behavioral genetic studies of the etiology of early reading development at specific time points. We found primarily genetic influences on individual differences at post–1st grade for all measures. Genetic influences on variance in growth rates were also found, with evidence of small, nonsignificant, shared environmental influences for two measures. We discuss our results, including their implications for educational policy.     

Genes, environment, and dyslexia the 2005 Norman Geschwind memorial lecture

This article presents an overview of some methods and results from our continuing studies of genetic and environmental influences on dyslexia, and on individual differences across the normal range that have been conducted over the past 25 years in the Colorado Learning Disabilities Research Center (CLDRC) and in related projects. CLDRC investigators compare the similarities of identical twin pairs who share all their genes and fraternal twins who share half their segregating genes to assess the balance of genetic, shared family environment, and nonshared environment influences on dyslexia and on individual differences across the normal range. We have learned that among the children we have studied in Colorado, group deficits in reading (dyslexia) and individual differences in reading across the normal range are primarily due to genetic influences, and these genetic influences are often shared with some of the same genetic influences on deficits and individual differences in language and ADHD. We have also learned from our molecular-genetic linkage studies that there are regions on several chromosomes likely to contain genes that influence dyslexia. Several specific genes within these regions have been tentatively identified through molecular-genetic association analyses, but much more research is needed to understand the pathways among specific genes, regions of noncoding DNA that regulate the activity of those genes, the brain, and dyslexia. I conclude with a discussion of our research on individual differences in early reading development, on the role of early learning constraints in dyslexia, and on how genetic influences are expressed through their interaction and correlation with the environment.     

Familial resemblance for measures of reading performance in families of reading-disabled and control twins

In order to assess familial resemblance for measures of reading performance, data from 314 pairs of twins in which at least one member of each pair is reading-disabled [142 monozygotic (MZ) and 172 dizygotic (DZ) twin pairs], 273 matched control pairs (131 MZ and 142 DZ pairs), and their parents were subjected to both correlation and regression analyses. Results indicate that parent-offspring resemblance in families of reading-disabled probands does not differ substantially from that in families of controls. In general, the correlations and regressions for MZ twin pairs are greater than those for DZ twins; thus, individual differences in reading performance are due at least in part to heritable influences. As expected, regression coefficients are consistently larger than correlation coefficients for both parent-offspring and proband-cotwin comparisons in the reading-disabled sample, illustrating that regression analyses are more appropriate than correlations for assessing familial resemblance in selected samples.     

Students' self-perception of reading ability,enjoyment of reading and reading achievement

Using data from New Zealand's National Education Monitoring Project, a light sampling, low stakes performance based national school assessment program, reading self-efficacy, reading enjoyment, and reading achievement were examined in samples of 8 and 12 year old children. Sample sizes were n = 480 for each group. While reading achievement increased substantially in going from age 8 to age 12, reading enjoyment and reading self-efficacy declined. Girls outperformed boys in reading achievement and showed higher levels of reading enjoyment; differences in self-efficacy by gender were minimal. Results are discussed in terms of previous research and implications for instructional practice.     

A factorial analysis of timed and untimed measures of mathematics and reading abilities in school aged twins

The present study examined the phenotypic and genetic relationship between fluency and non-fluency-based measures of reading and mathematics performance. Participants were drawn from the Western Reserve Reading and Math Project, an ongoing longitudinal twin project of same-sex MZ and DZ twins from Ohio. The present analyses are based on tester-administered measures available from 228 twin pairs (age M = 9.86 years). Measurement models suggested that four factors represent the data, namely Decoding, Fluency, Comprehension, and Math. Subsequent quantitative genetic analyses of these latent factors suggested that a single genetic factor accounted for the covariance among these four latent factors. However, there were also unique genetic effects on Fluency and Math, independent from the common genetic factor. Thus, although there is a significant genetic overlap among different reading and math skills, there may be independent genetic sources of variation related to measures of decoding fluency and mathematics.     

Genetic and environmental influences on rapid naming and reading ability: A twin study

The present study assesses the genetic and environmental etiologies of reading, rapid naming (RN), and their covariation by fitting multivariate structural equation models to data from 587 twin pairs in which at least one member of the pair exhibited reading difficulties (low-range) and from 360 control (normal-range) twin pairs who were tested in the Colorado Learning Disabilities Research Center. Results from a bivariate phenotypic analysis with two hypothesized latent factors, READ and RN, indicated that the correlation between reading and rapid naming performance for the low-range sample was significantly higher than that of the normal-range sample. When this model was partitioned to include estimates of genetic, shared environmental, and nonshared environmental influences, resulting heritability estimates did not differ significantly for the low-range and normal-range samples for either READ or RN. However, similar to the phenotypic correlation, the genetic correlation between the READ and RN latent factors could not be equated for the two groups. Thus, the etiology of the relationship between reading performance and rapid naming may differ for children with reading difficulties and normally-achieving readers. Moreover, these results support previous findings that the best predictors of reading skills may differ for samples of children with normal reading levels and those with reading difficulties.     

Genetic Influences on Activity Level in Early Childhood: Do Situations Matter?

Although genetic influences on individual differences in activity level (AL) are well documented, few studies have considered the etiology of AL from a contextual perspective. In the present study, cross-situational and context-specific genetic effects on individual differences in AL at age 2 were examined. The AL of 312 twin pairs (144 monozygotic and 168 dzygotic) was mechanically assessed with actigraphs in the home and in laboratory test and play situations. AL displayed significant genetic variance in all 3 situations. Moreover, actigraph scores significantly correlated across situations. Multivariate genetic model-fitting analyses found that the observed cross-situational continuity in AL was due entirely to genetic factors. Situational differences in AL arise from genetic, shared environmental, and nonshared environmental influences.     

Cognitive modelling and the behaviour genetics of reading

While it is well known that reading is highly heritable, less has been understood about the bases of these genetic influences. In this paper, we review the research that we have been conducting in recent years to examine genetic and environmental influences on the particular reading processes specified in the dual‐route cognitive model of reading. We argue that a detailed understanding of the role of genetic factors in reading acquisition requires the delineation and measurement of precise phenotypes, derived from well‐articulated models of the reading process. We report evidence for independent genetic influences on the lexical and nonlexical reading processes represented in the dual‐route model, based on studies of children with particular subtypes of dyslexia, and on univariate and multivariate genetic modelling of reading performance in the normally reading population.     

Children learning to read later catch up to children reading earlier

Two studies from English-speaking samples investigated the methodologically difficult question of whether the later reading achievement of children learning to read earlier or later differs. Children (n = 287) from predominantly state-funded schools were selected and they differed in whether the reading instruction age (RIA) was either five or seven years. Study 1 covered the first six years of school following three cohorts across a two-year design. Analyses accounted for receptive vocabulary, reported parental income and education, school-community affluence, classroom instruction, home literacy environment, reading self-concept, and age. The earlier RIA group had initially superior letter naming, non-word, word, and passage reading but this difference in reading skill disappeared by age 11. In Study 2, the decoding, fluency, and reading comprehension performance of 83 additional middle school-age children was compared. The two groups exhibited similar reading fluency, but the later RIA had generally greater reading comprehension. Given that the design was non-experimental, we urge further research to better understand developmental patterns and influences arising from different RIAs.     

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.     

Genetic and environmental etiologies of reading difficulties: DeFries-Fulker analysis of reading performance data from twin pairs and their nontwin siblings

Reading performance data from 254 pairs of identical (MZ) and 420 pairs of fraternal (DZ) twins, 8.0 to 20.0 years of age, were subjected to multiple regression analyses. An extension of the DeFries-Fulker (DF) analysis (DeFries & Fulker, 1985, 1988) that facilitated inclusion of data from 303 of their nontwin siblings was employed. In addition to providing estimates of heritability, this analysis yields a test of the difference between shared environmental influences for twins versus siblings (Astrom et al., 2011). Results suggest that proband reading deficits are due substantially to genetic factors (.67 ± .07, p < .001), and that shared environmental influences are significantly higher for members of twin pairs than for those of twins and their nontwin siblings (viz., .25 versus .17, p = .02).     

Genetic and environmental influences on writing and their relations to language and reading

Identical and fraternal twins (N?=?540, age 8 to 18 years) were tested on three different measures of writing (Woodcock-Johnson III Tests of Achievement—Writing Samples and Writing Fluency; Handwriting Copy from the Group Diagnostic Reading and Aptitude Achievement Tests), three different language skills (phonological awareness, rapid naming, and vocabulary), and three different reading skills (word recognition, spelling, and reading comprehension). Substantial genetic influence was found on two of the writing measures, writing samples and handwriting copy, and all of the language and reading measures. Shared environment influences were generally not significant, except for Vocabulary. Non-shared environment estimates, including measurement error, were significant for all variables. Genetic influences among the writing measures were significantly correlated (highest between the speeded measures writing fluency and handwriting copy), but there were also significant independent genetic influences between copy and samples and between fluency and samples. Genetic influences on writing were significantly correlated with genetic influences on all of the language and reading skills, but significant independent genetic influences were also found for copy and samples, whose genetic correlations were significantly less than 1.0 with the reading and language skills. The genetic correlations varied significantly in strength depending on the overlap between the writing, language, and reading task demands. We discuss implications of our results for education, limitations of the study, and new directions for research on writing and its relations to language and reading.