Response to early literacy instruction in the United States, Australia, and Scandinavia A behavioral-genetic analysis

Abstract Genetic and environmental influences on early reading and spelling at the end of kindergarten and Grade 1 were compared across three twin samples tested in the United States, Australia, and Scandinavia. Proportions of variance due to genetic influences on kindergarten reading were estimated at .84 in Australia, .68 in the U.S., and .33 in Scandinavia. The effects of shared environment on kindergarten reading were estimated at .09 in Australia, .25 in the U.S., and .52 in Scandinavia. A similar pattern of genetic and environmental influences was obtained for kindergarten spelling. One year later when twins in all three samples had received formal literacy instruction for at least one full school year, heritability was similarly high across country, with estimated genetic influences varying between .79 and .83 for reading and between .62 and .79 for spelling. These findings indicate that the pattern of genetic and environmental influences on early reading and spelling development varies according to educational context, with genetic influence increasing as a function of increasing intensity of early instruction. Longitudinal analyses revealed genetic continuity for both reading and spelling between kindergarten and Grade 1 across country. However, a new genetic factor comes into play accounting for independent variance in reading at Grade 1 in the U.S. and Scandinavia, suggesting a change in genetic influences on reading. Implications for responseto-instruction are discussed. 4 Historically, and as late as 1800, more than 50% of the population in most western countries was illiterate. The opportunity to learn to read and write was a privilege, to a large extent determined by social-cultural conditions In this article, we continue to report on our International Longitudinal Twin Study (ILTS) of early language and literacy development The main purpose here was to compare genetic and environmental influences on early reading and spelling skills across three twin samples tested in the United States, Australia, and Scandinavia (i.e., Sweden and Norway) and across time of testing (i.e., kindergarten and Grade 1). Two questions are addressed: First, are there any differences in the pattern of genetic and environmental influences on early reading and spelling skills across country? Second, what are the changes in the pattern of genetic and environmental influences on reading and spelling from kindergarten to Grade 1? The general expectation is that the effects of environment on literacy skills should decrease and the genetic contribution increase as a function of intensity and consistency of instruction, across countries and time. This approach should also inform recent interest in response-to-intervention, or RTI, as a method to ascertain, define, and remediate 5 reading difficulties The ILTS has previously documented substantial effects of genes and relatively minor However, in a recent ILTS study of data collected near the end of kindergarten ), individual differences in reading and spelling skills were mainly 6 accounted for by genetic factors in a sample of Australian twins, with estimates of .91 and .84, respectively. In contrast, in a sample of U.S. twins from the state of Colorado, only approximately half of the variance in reading and spelling was accounted for by genetic influences, with the other half attributed to shared and non-shared environment. Although these country differences in genetic and environmental influences on individual differences in reading and spelling were not statistically significant with the available sample sizes, we hypothesized that the trends might be explained by country differences in educational practice. Compulsory school starts at around age five in both Australia and Colorado, but in New South Wales, Australian children enter a school system regulated by a state-wide curriculum mandating that at least 35% of a full school week (9 am to 3 pm, five days a week) should be devoted to language and literacy instruction. In contrast, in Colorado children attend kindergarten school for only 3-4 hours each day, and there is no state-mandated curriculum for teaching reading and spelling. One plausible explanation for the different pattern of genetic and environmental influences on reading and spelling in Australia and U.S. is that a state-wide curriculum emphasizing intense literacy instruction reduces the environmental range in the population, and thus, the amount of variance in reading and spelling skills that can be accounted for by environmental factors. Another explanation is that the greater intensity of instruction in NSW engages genetically-influenced learning processes earlier than in the US, resulting in a higher genetic contribution to overall variability. To further explore these hypotheses, the present study includes a sample of Scandinavian twins. In Scandinavia, compulsory school starts when the child is seven years old, that is, one to two years later compared to Australia and the U.S. Nevertheless, almost all children do attend kindergarten prior to compulsory attendance in Grade 1, but kindergarten curriculum in Sweden and Norway emphasizes social, emotional, and aesthetic development rather than early literacy acquisition. In this way, Scandinavia represents a population where environmental 7 variation outside of school might have a substantial impact on individual differences in kindergarten reading and spelling skills because there is no formal reading instruction in kindergarten. Instead, literacy socialization is mainly given informally at home. However, at seven years of age, in Grade 1, teaching reading and spelling is the target activity in school, and literacy instruction is guided by a master plan common to all schools in Sweden and Norway. This change from informal literacy teaching taking place at the children's home to a countrywide curriculum emphasizing formal reading and spelling instruction should reduce environmental range and increase the intensity of engagement. From kindergarten to Grade 1, we hypothesize, therefore, that the heritability of literacy skills increases and the importance of shared environment decreases in Scandinavia. To summarize, we hypothesize different contributions from genes and environment to kindergarten literacy skills across countries. We also hypothesize an increase in genetic effects on literacy from kindergarten to Grade 1, especially in Scandinavia where formal reading instruction is introduced one year later than in Australia and the U.S. These questions are addressed in the present study through univariate behavior-genetic analyses of data from identical and same-sex fraternal twins tested near the end of kindergarten and first grade. In addition to comparing the magnitudes of genetic and environmental influences between countries and grades in univariate analyses, with a multivariate approach we also address the question whether the same or different sources of genetic and environmental influences account for individual differences in literacy at kindergarten and Grade 1. Based on the differences in the curriculum for literacy instruction across countries summarized above, we hypothesize continuity in the pattern of genetic and environmental influences on reading and spelling from kindergarten to Grade 1 in Australia, but a possible change in genetic and environmental effects on literacy skills in Scandinavia, with the US representing an intermediate case. Method Participants The kindergarten sample comprised a total of 812 same-sex twin pairs recruited from the Colorado Twin Registry in the U.S., the National Heath and Medical Research Council's Australian Twin Registry, and from the Medical Birth Registries in Norway and Sweden (see Actual attrition because of families leaving the project is virtually zero. Only participants for whom the predominant language of their country (i.e., English, Swedish, or Norwegian) was the first language spoken at home were selected. There were no significant differences in parents' mean years of education across twin samples. Also, the means were around 14 years suggesting that level of education is representative for each country. Zygosity was determined by DNA analysis from cheek swab collection, or, in a minority of cases, by selected items from the questionnaire by Literacy skills Reading. Reading skills in kindergarten and Grade 1 were measured by both the word and nonword subtests from the Test of Word Reading Efficiency (TOWRE; Torgesen, Wagner, & Rashotte, 1999), with both Forms A and B administered and averaged to increase reliability (test-retest reliability for children aged 6-9 years, .97 for word and .90 for nonword standard scores). In each Form, children read a list of words and a list of nonwords as quickly as possible in 45 sec. A composite measure of reading skill was created for phenotypic analyses, justified by high correlations, .83 and .86 on average, between word and nonword reading at both kindergarten and at the end of Grade 1. For the behavior genetic analyses, we modelled the four subtests of word and nonword reading as latent traits. Spelling. At kindergarten, spelling was measured by a test developed by Byrne and Fielding- Procedure Children were assessed individually by trained examiners in their homes and/or schools at the end of kindergarten and Grade 1. To foster fidelity of assessment between testers and sites, we have adopted the practice of videotaping samples of test sessions and having each tester inspect the tapes of other testers. In this study, we only report on reading and spelling measures performed at each age. However, in single one-hour sessions at each of kindergarten and Grade 1, several other measures such as phonological awareness, RAN, and verbal abilities were included (for details, see Analysis One-way analyses of variance (ANOVA) and Tukey HSD post hoc tests were performed to test differences between country samples for reading and spelling at kindergarten and Grade 1. The magnitude of the mean differences was calculated using Cohen's d. Genetic and environmental influences on reading and spelling skills across country and within country 11 across time were analyzed using monozygotic (MZ) and dizygotic (DZ) twin correlations, and models were fitted from raw data using maximum likelihood estimation in Mx Results Reading and spelling skills across country Means and standard deviations for reading and spelling at kindergarten and Grade 1 across country and effect size estimations for mean differences are presented in Behavior-genetic analyses Standardized raw data adjusted for age and gender effects within each twin sample were used as input for all behavior-genetic analyses. To estimate the relative influence on individual differences from additive genetic effects (a 2 ), shared-environment effects (c 2 ), and nonsharedenvironment effects (e 2 ), the data for reading and spelling skills were subjected to structural equation modelling by use of the Mx statistical modelling package (Neale, Boker, Xie, & Maes, 2002). In this section, we start by presenting correlations between MZ and DZ twins

Genome-wide association scan identifies new variants associated with a cognitive predictor of dyslexia

Developmental dyslexia (DD) is one of the most prevalent learning disorders, with high impact on school and psychosocial development and high comorbidity with conditions like attention-deficit hyperactivity disorder (ADHD), depression, and anxiety. DD is characterized by deficits in different cognitive skills, including word reading, spelling, rapid naming, and phonology. To investigate the genetic basis of DD, we conducted a genome-wide association study (GWAS) of these skills within one of the largest studies available, including nine cohorts of reading-impaired and typically developing children of European ancestry (N = 2562-3468). We observed a genome-wide significant effect (p <1 x 10(-8)) on rapid automatized naming of letters (RANlet) for variants on 18q12.2, within MIR924HG (micro-RNA 924 host gene; rs17663182 p = 4.73 x 10(-9)), and a suggestive association on 8q12.3 within NKAIN3 (encoding a cation transporter; rs16928927, p = 2.25 x 10(-8)). rs17663182 (18q12.2) also showed genome-wide significant multivariate associations with RAN measures (p = 1.15 x 10(-8)) and with all the cognitive traits tested (p = 3.07 x 10(-8)), suggesting (relational) pleiotropic effects of this variant. A polygenic risk score (PRS) analysis revealed significant genetic overlaps of some of the DD-related traits with educational attainment (EDUyears) and ADHD. Reading and spelling abilities were positively associated with EDUyears (p similar to [10(-5)-10(-7)]) and negatively associated with ADHD PRS (p similar to [10(-8)-10(-17)]). This corroborates a long-standing hypothesis on the partly shared genetic etiology of DD and ADHD, at the genome-wide level. Our findings suggest new candidate DD susceptibility genes and provide new insights into the genetics of dyslexia and its comorbities.Peer reviewe

Genome-wide association study reveals new insights into the heritability and genetic correlates of developmental dyslexia

Developmental dyslexia (DD) is a learning disorder affecting the ability to read, with a heritability of 40-60%. A notable part of this heritability remains unexplained, and large genetic studies are warranted to identify new susceptibility genes and clarify the genetic bases of dyslexia. We carried out a genome-wide association study (GWAS) on 2274 dyslexia cases and 6272 controls, testing associations at the single variant, gene, and pathway level, and estimating heritability using single-nucleotide polymorphism (SNP) data. We also calculated polygenic scores (PGSs) based on large-scale GWAS data for different neuropsychiatric disorders and cortical brain measures, educational attainment, and fluid intelligence, testing them for association with dyslexia status in our sample. We observed statistically significant (p <2.8 x 10(-6)) enrichment of associations at the gene level, forLOC388780(20p13; uncharacterized gene), and forVEPH1(3q25), a gene implicated in brain development. We estimated an SNP-based heritability of 20-25% for DD, and observed significant associations of dyslexia risk with PGSs for attention deficit hyperactivity disorder (atp(T) = 0.05 in the training GWAS: OR = 1.23[1.16; 1.30] per standard deviation increase;p = 8 x 10(-13)), bipolar disorder (1.53[1.44; 1.63];p = 1 x 10(-43)), schizophrenia (1.36[1.28; 1.45];p = 4 x 10(-22)), psychiatric cross-disorder susceptibility (1.23[1.16; 1.30];p = 3 x 10(-12)), cortical thickness of the transverse temporal gyrus (0.90[0.86; 0.96];p = 5 x 10(-4)), educational attainment (0.86[0.82; 0.91];p = 2 x 10(-7)), and intelligence (0.72[0.68; 0.76];p = 9 x 10(-29)). This study suggests an important contribution of common genetic variants to dyslexia risk, and novel genomic overlaps with psychiatric conditions like bipolar disorder, schizophrenia, and cross-disorder susceptibility. Moreover, it revealed the presence of shared genetic foundations with a neural correlate previously implicated in dyslexia by neuroimaging evidence.Peer reviewe

Response to early literacy instruction in the United States, Australia, and Scandinavia : A behavioral-genetic analysis

Genetic and environmental influences on early reading and spelling at the end of kindergarten and Grade 1 were compared across three twin samples tested in the United States, Australia, and Scandinavia. Proportions of variance due to genetic influences on kindergarten reading were estimated at .84 in Australia, .68 in the U.S., and .33 in Scandinavia. The effects of shared environment on kindergarten reading were estimated at .09 in Australia, .25 in the U.S., and .52 in Scandinavia. A similar pattern of genetic and environmental influences was obtained for kindergarten spelling. One year later when twins in all three samples had received formal literacy instruction for at least one full school year, heritability was similarly high across country, with estimated genetic influences varying between .79 and .83 for reading and between .62 and .79 for spelling. These findings indicate that the pattern of genetic and environmental influences on early reading and spelling development varies according to educational context, with genetic influence increasing as a function of increasing intensity of early instruction. Longitudinal analyses revealed genetic continuity for both reading and spelling between kindergarten and Grade 1 across country. However, a new genetic factor comes into play accounting for independent variance in reading at Grade 1 in the U.S. and Scandinavia, suggesting a change in genetic influences on reading. Implications for response-to-instruction are discussed.Original publication: Stefan Samuelsson, Brian Byrne, Richard K. Olson, Jacqueline Hulslander, Sally Wadsworth, Robin Corley, Erik G. Willcutt and John C. DeFries, Response to early literacy instruction in the United States, Australia, and Scandinavia: A behavioral-genetic analysis, 2008, Learning and Individual Differences, (18), 3, 289-295.http://dx.doi.org/10.1016/j.lindif.2008.03.004. Copyright: Elsevier B.V., http://www.elsevier.com

The Genetic and Environmental Etiologies of the Relations between Cognitive Skills and Components of Reading ability.

Although previous research has shown cognitive skills to be important predictors of reading ability in children, the respective roles for genetic and environmental influences on these relations is an open question. The present study explored the genetic and environmental etiologies underlying the relations between selected executive functions and cognitive abilities (working memory, inhibition, processing speed, and naming speed) with 3 components of reading ability (word reading, reading comprehension, and listening comprehension). Twin pairs drawn from the Colorado Front Range (n = 676; 224 monozygotic pairs; 452 dizygotic pairs) between the ages of 8 and 16 (M = 11.11) were assessed on multiple measures of each cognitive and reading-related skill. Each cognitive and reading-related skill was modeled as a latent variable, and behavioral genetic analyses estimated the portions of phenotypic variance on each latent variable due to genetic, shared environmental, and nonshared environmental influences. The covariance between the cognitive skills and reading-related skills was driven primarily by genetic influences. The cognitive skills also shared large amounts of genetic variance, as did the reading-related skills. The common cognitive genetic variance was highly correlated with the common reading genetic variance, suggesting that genetic influences involved in general cognitive processing are also important for reading ability. Skill-specific genetic variance in working memory and processing speed also predicted components of reading ability. Taken together, the present study supports a genetic association between children\u27s cognitive ability and reading ability

The genetic and environmental etiologies of individual differences in early reading growth in Australia, the United States, and Scandinavia

This first cross-country twin study of individual differences in reading growth from post-kindergarten to post-second grade analyzed data from 487 twin pairs from the United States, 267 twin pairs from Australia, and 280 twin pairs from Scandinavia. Data from two reading measures were fit to biometric latent growth models. Individual differences for the reading measures at post-kindergarten in the United States and Australia were due primarily to genetic influences and to both genetic and shared environmental influences in Scandinavia. In contrast, individual differences in growth generally had large genetic influences in all countries. These results suggest that genetic influences are largely responsible for individual differences in early reading development. In addition, the timing of the start of formal literacy instruction may affect the etiology of individual differences in early reading development but have only limited influence on the etiology of individual differences in growth

Genetic and Environmental Etiologies of the Longitudinal Relations Between Prereading Skills and Reading

The present study explored the environmental and genetic etiologies of the longitudinal relations between prereading skills and reading and spelling. Twin pairs (n = 489) were assessed before kindergarten (M = 4.9 years), post‐first grade (M = 7.4 years), and post‐fourth grade (M = 10.4 years). Genetic influences on five prereading skills (print knowledge, rapid naming, phonological awareness, vocabulary, and verbal memory) were primarily responsible for relations with word reading and spelling. However, relations with post‐fourth‐grade reading comprehension were due to both genetic and shared environmental influences. Genetic and shared environmental influences that were common among the prereading variables covaried with reading and spelling, as did genetic influences unique to verbal memory (only post‐fourth‐grade comprehension), print knowledge, and rapid naming

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