Longitudinal twin study of early reading development in three countries: Preliminary results

We have initiated parallel longitudinal studies in Australia (Byrne, PI), the United States (Olson, PI), and Norway (Samuelsson, PI) of identical and fraternal twins who are being tested in preschool for prereading skills, and in kindergarten, first grade, and second grade for the development of early reading, spelling, and related cognitive skills. Comparisons of the similarities of identical and fraternal twins will reveal the relative influence of genetic, shared family environment, and nonshared environment on individual differences at and across different stages of development. Family and twin-specific environmental information is also being directly assessed through parent questionnaires and observations by testers. Most of the data collected so far have been from preschool twins (146 in Australia, 284 in the United States, and 70 in Norway). Preliminary analyses for the preschool cognitive measures showed reliable genetic influences on phonological awareness and several measures of memory and learning. In contrast, vocabulary, grammar, and morphology showed significant shared environment and negligible genetic effects. A print knowledge composite showed both genetic and shared environment influence.     

Dynamic and static assessment of phonological awareness in preschool: a behavior-genetic study

The genetic and environmental overlap between static and dynamic measures of preschool phonological awareness (PA) and their relation to preschool letter knowledge (LK) and kindergarten reading were examined using monozygotic and dizygotic twin children (maximum N = 1,988). The static tests were those typically used to assess a child's current level of PA such as blending and elision, and the dynamic test included instruction in phoneme identity to assess the child's ability to respond to this instruction. Both forms were influenced by genes and by shared and nonshared environment. The static and dynamic versions were influenced by the same genes, and part of the total genetic influence was shared with LK. They were subject to both overlapping and independent shared environment influences, with the component in common also affecting LK. Nonshared environment influences were mostly independent. Scores from dynamic assessment added only minimally to variance explained in kindergarten reading after LK and static assessment had been factored in. Although one of the genetic factors that influenced both forms of PA also affected kindergarten reading, it was only the one shared with LK. The authors conclude that dynamic assessment of PA in preschool offers little advantage over the more commonly used static forms, especially if LK scores are available, although they acknowledge its potential in cases of preschool educational disadvantage.     

Genetic and environmental influences on prereading skills and early reading and spelling development in the United States, Australia, and Scandinavia

Genetic and environmental influences on prereading skills in preschool and on early reading and spelling development at the end of kindergarten were compared among samples of identical and fraternal twins from the U.S. (Colorado), Australia, and Scandinavia. Mean comparisons revealed significantly lower preschool print knowledge in Scandinavia, consistent with the relatively lower amount of shared book reading and letter-based activities with parents, and lack of emphasis on print knowledge in Scandinavian preschools. The patterns of correlations between all preschool environment measures and prereading skills within the samples were remarkably similar, as were the patterns of genetic, shared environment, and non-shared environment estimates: in all samples, genetic influence was substantial and shared environment influence was relatively weak for phonological awareness, rapid naming, and verbal memory; genetic influence was weak, and shared environment influence was relatively strong for vocabulary and print knowledge. In contrast, for reading and spelling assessed at the end of kindergarten in the Australian and U.S. samples, there was some preliminary evidence for country differences in the magnitude of genetic and environmental influences. We argue that the apparently higher genetic and lower shared environment influence in the Australian sample was related to a greater emphasis on formal reading instruction, resulting in more advanced reading and spelling skills at the end of kindergarten, and thus there was greater opportunity to observe genetic influences on response to systematic reading instruction among the Australian twins.     

Genetic and environmental influences on sex-typed behavior during the preschool years

The genetic and environmental etiologies of sex-typed behavior were examined during the preschool years in a sample of 3,990 three- to four-year-old twin and non-twin sibling pairs. Results showed moderate genetic and significant shared environmental influence for boys and substantial genetic and moderate shared environmental influence for girls. For both boys and girls, twin-specific shared environmental effects contributed to twins' similarity in gender role behavior and accounted for approximately 22% of the shared environmental variance. These findings extend previous research conducted with older samples by showing not only important genetic contributions to gender role behavior but also an important role for shared environment. The inclusion of non-twin siblings showed that some of the shared environmental influence is specific to twins.     

Little Evidence That Socioeconomic Status Modifies Heritability of Literacy and Numeracy in Australia

Socioeconomic status (SES) has been found to moderate the influence of genes and the environment on cognitive ability, such that genetic influence is greater when SES is higher, and the shared environment is greater when SES is lower, but not in all Western countries. The effects of both family and school SES on the heritability of literacy and numeracy in Australian twins aged 8, 10, 12, and 14 years with 1,307, 1,235, 1,076, and 930 pairs at each age, respectively, were tested. Shared environmental influences on Grade 3 literacy were greater with low family SES, and no other moderating effects of SES were significant. These findings are contrasted with results from the United States and the United Kingdom.     

The genetic and environmental origins of learning abilities and disabilities in the early school years

Despite the importance of learning abilities and disabilities in education and child development, little is known about their genetic and environmental origins in the early school years. We report results for English (which includes reading, writing, and speaking), mathematics, and science as well as general cognitive ability in a large and representative sample of U.K. twins studied at 7, 9, and 10 years of age. Although preliminary reports of some of these data have been published, the purpose of this monograph is to present new univariate, multivariate, and longitudinal analyses that systematically examine genetic and environmental influences for the entire sample at all ages for all measures for both the low extremes (disabilities) and the entire sample (abilities). English, mathematics, and science yielded similarly high heritabilities and modest shared environmental influences at 7, 9, and 10 years despite major changes in content across these years. We draw three conclusions that go beyond estimating heritability. First, the abnormal is normal: Low performance is the quantitative extreme of the same genetic and environmental influences that operate throughout the normal distribution. Second, continuity is genetic and change is environmental: Longitudinal analyses suggest that age-to-age stability is primarily mediated genetically, whereas the environment contributes to change from age to age. Third, genes are generalists and environments are specialists: Multivariate analyses indicate that genes largely contribute to similarity in performance within and between the three domains--and with general cognitive ability--whereas the environment contributes to differences in performance. These conclusions have far-reaching implications for education and child development as well as for molecular genetics and neuroscience.     

Genetic and environmental influences on early literacy

Prereading and early reading skills of preschool twin children in Australia, Scandinavia and the United States were explored in a genetically sensitive design (max. N=627 preschool pairs and 422 kindergarten pairs). Analyses indicated a strong genetic influence on preschool phonological awareness, rapid naming and verbal memory. Print awareness, vocabulary and grammar/morphology were subject primarily to shared environment effects. There were significant genetic and shared environment correlations among the preschool traits. Kindergarten reading, phonological awareness and rapid naming were primarily affected by genes, and spelling was equally affected by genes and shared environment. Multivariate analyses revealed genetic and environmental overlap and independence among kindergarten variables. Longitudinal analyses showed genetic continuity as well as change in phonological awareness and rapid naming across the 2 years. Relations among the preschool variables of print awareness, phonological awareness and rapid naming and kindergarten reading were also explored in longitudinal analyses. Educational implications are discussed.     

The etiology of giftedness

Many theories of giftedness either explicitly or implicitly acknowledge the role of genetic influences; yet, empirical work has not been able to establish the impact that genes have specifically on gifted behavior. In contrast, a great deal of research has been targeted at understanding the etiology of individual differences in general and specific cognitive abilities across the entire range of ability and to a lesser extent, high cognitive ability. This paper attempts to outline what we know and what we don't know about the etiology of giftedness as operationally defined as high g. We review studies selected to represent a variety of approaches that each address a different question about genetics and giftedness. These studies include quantitative genetic research which estimate heritability, shared and nonshared family environment – at the high and low ends of intelligence – as well as the heritability of group differences for general cognitive ability and specific cognitive abilities. We discuss the molecular genetic methods and mechanisms contributing to cognitive ability and suggest how epigenetic factors may operate. Quantitative and molecular genetic studies that include endophenotypes representing intelligence at a level closer to the genotype, are also included. This last group of studies represent a relatively new area of work that builds on and extends the extensive groundwork established by classic quantitative genetic studies of behavior.     

Socioeconomic status impacts genetic influences on the longitudinal dynamic relationship between temperament and general cognitive ability in childhood: The Louisville Twin Study

The current analysis investigates genetic and environmental influences on the bidirectional relationships between temperament and general cognitive ability (GCA). Measures of GCA and three temperament factors (persistence, approach, and reactivity) were collected from 486 children ages 4–9 years (80% white, 50% female) from the Louisville Twin Study from 1976 to 1998. The results indicated a bidirectional dynamic model of temperament influencing subsequent GCA and GCA influencing subsequent temperament. The dynamic relationship between temperament and GCA arose primarily from shared genetic variance, particularly in families with higher socioeconomic status, where input from temperament contributed on average 20% to genetic variance in GCA versus 0% in lower SES families.     

A genetically sensitive investigation of the effects of the school environment and socio‐economic status on academic achievement in seven‐year‐olds

Although it is well established that school characteristics (SCH) and socio‐economic status (SES) are associated with academic achievement (ACH), these correlations are not necessarily causal. Because academic achievement shows substantial genetic influence, it is useful to embed such investigations in genetically sensitive designs in order to examine environmental influences more precisely by controlling genetic influence on ACH. In the first study of this kind for academic achievement, data were collected for 1,063 same‐sex pairs of seven‐year‐old MZ and DZ twins for teacher‐assessed ACH, UK statistics on SCH, and parent‐reported SES. Exclusive of genetic influence on school achievement, shared environment (environmental influences that make siblings similar) accounts for 12% of the variance in academic achievement. SCH accounts for 17% and SES accounts for 83% of this shared environmental variance. Exclusive of genetic and shared environmental influence including SCH and SES, nonshared environment (environmental influences that do not make siblings similar) accounts for 19% of the variance in academic achievement. The importance of nonshared environmental influences on academic achievement leads to the question of what these child‐specific experiences might be that are not shared by children in the same family, school, and classroom.     

The Etiology of Science Performance: Decreasing Heritability and Increasing Importance of the Shared Environment From 9 to 12 Years of Age

During childhood and adolescence, increases in heritability and decreases in shared environmental influences have typically been found for cognitive abilities. A sample of more than 2,500 pairs of twins from the Twins Early Development Study was used to investigate whether a similar pattern would be found for science performance from 9 to 12 years. Science performance was based on teacher-assessed U.K. National Curriculum standards. Science at 9 years showed high heritability (64%) and modest shared environmental (16%) estimates. In contrast to the expected developmental pattern, heritability was significantly lower at 12 years (47%) and shared environmental influences were significantly higher (32%). Understanding what these increasingly important shared environmental influences are could lead to interventions that encourage engagement in science throughout the lifespan.     

A Twin Study of Attachment in Preschool Children

The degree to which individual differences in child-parent attachment were mediated by genetic, shared environmental, and nonshared environmental influences was investigated. One hundred and ten preschool-aged twin pairs (N = 220) were assessed in the Strange Situation and coded using conventional four-way classifications and a continuous measure of attachment security. The degree of sibling similarity in attachment was substantial, with an overall concordance rate of 67% at the secure/insecure level. The degree of concordance was equally high in monozygotic and dizygotic twin pairs, 70% and 64%, respectively, suggesting little genetic influence but a moderate degree of discordance. Twin similarity on the continuous measure of attachment security was r(57) = .48 and r(53) = .38 for monozygotic and dizygotic twin pairs, respectively, also consistent with a modest role for genetic influence but a significant effect for shared and nonshared environment. The implications for genetic influences on the environment and for understanding nonshared and shared environmental influences are discussed.     

Genetic and Environmental Influences on Perceptions of Self-Worth and Competence in Adolescence: A Study of Twins, Full Siblings, and Step-Siblings

Although it is generally assumed that the origins of adolescents' perceptions of self-competence lie in shared family environmental influences, the contributions of nonshared environmental or genetic influences have not been explored. We investigated sibling resemblance for perceived competence and self-worth in 720 adolescent pairs aged 10 to 18 years, using a twin, full sibling, and step-sibling design. Our goals were to assess the magnitude of shared and nonshared environmental influences and to disentangle resemblance due to shared genetic heritage from that due to shared environmental experiences. Shared environment was not significant for any of the scales. 4 of the subscales showed significant genetic influence: scholastic, social, physical, and athletic competence. We also explored possible sources of genetic influences on perceived competence. Bivariate models revealed common genetic variance between scholastic competence and vocabulary and social competence and sociability. These measures, however, did not account for all of the genetic variance in perceived social and scholastic competence.     

The genetic-environmental etiology of cognitive school readiness and later academic achievement in early childhood

Using a genetic design of 840 60-month-old twins, this study investigated the genetic and environmental contributions to (a) individual differences in four components of cognitive school readiness, (b) the general ability underlying these four components, and (c) the predictive association between school readiness and school achievement. Results revealed that the contribution of the shared environment for cognitive school readiness was substantial. Genetic effects were more important for the core abilities underlying school readiness than for each specific skill, although shared environment remained the largest factor overall. Genetic, shared, and nonshared environmental factors all accounted for the predictive association between school readiness and early school achievement. These results contribute to a better understanding of the early determinants of school readiness.     

Are Head Start Effects Sustained? A Longitudinal Follow-up Comparison of Disadvantaged Children Attending Head Start, No Preschool, and Other Preschool Programs

This study investigates the sustained effects into kindergarten and grade 1 of Project Head Start for disadvantaged black children. Participation in generic Head Start programs was compared to both no preschool and other preschool experience for disadvantaged children in two American cities in 1969-1970. Incorporating both pretest/posttest and comparison group information, the study has advantages over other Head Start impact studies. Both preprogram background and cognitive differences were controlled in a covariance analysis design, using dependent measures in the cognitive, verbal, and social domains. Children who attended Head Start maintained educationally substantive gains in general cognitive/analytic ability, especially when compared to children without preschool experience. These effects were not as large as those found immediately following the Head Start intervention. Findings suggest an effect of preschool rather than of Head Start per se. Initial findings of greater effectiveness of Head Start for children of below average initial ability were reduced but not reversed. The diminution of effects over time, especially for low-ability children, may reflect differences in quality of subsequent schooling or home environment.     

The Interaction of Early Maternal Responsiveness and Children's Cognitive Abilities on Later Decoding and Reading Comprehension Skills

This longitudinal study evaluated the extent to which maternal responsiveness across early childhood and children's cognitive skills predicted children's 8-year decoding and reading comprehension skills for children who varied in biological risk (term, n = 83; preterm, n = 155). Patterns of maternal responsiveness during infancy (6, 12, and 24 months) and preschool (3 and 4 years) revealed 4 maternal clusters that varied in consistency and level of maternal responsiveness. Although not predictive of decoding skills, the interaction between children's 4-year cognitive ability and maternal responsiveness cluster predicted children's reading comprehension skills at 8 years of age, regardless of risk. Although consistently high levels of maternal responsive parenting across early childhood related to literacy outcomes for all children in the study, responsive parenting had a stronger relation to later reading comprehension skills for children with lower cognitive abilities, particularly when mothers demonstrated high responsiveness in children's infancy.     

Impact of Professional Development on Preschool Teachers’ Print References During Shared Read Alouds: A Latent Growth Curve Analysis

Abstract

This large-scale, experimental study aimed to (a) describe the extent to which teachers of preschool children at risk for reading difficulties make references to print during whole-class, shared-book read alouds and (b) empirically test the extent to which participation in professional development influences teachers’ frequency of references to print during shared read alouds across the academic year. Eighty-five preschool teachers working in targeted-enrollment preschool programs were randomly assigned to receive professional development aimed at increasing their use of print referencing or to a comparison condition involving alternative training; all teachers implemented a shared read aloud program in their classrooms. Implementation was documented via twice-monthly videotaping of shared read aloud sessions. Videos were coded for the frequency of print references and analyzed using latent growth curves. Despite considerable variability in teachers’ use of print referencing, findings showed sustained, meaningful changes in teachers’ use of print referencing with results favoring those who experienced the experimental professional development. Implications concerning effective professional development for supporting preschool children's emergent literacy skills are discussed.     

Preschool and primary school influences on the development of children's early numeracy skills between the ages of 3 and 7 years in Germany

Few studies have investigated how preschool and primary school interact to influence children's cognitive development. The present investigation explores German children's numeracy skills between age 3 (1st year of preschool) and age 7 (1st year of primary school). We first identified the influence of preschool experience on development while controlling for child factors, family background, and the quality of the home learning environment (HLE). We then considered how the instructional quality of primary schools influences numeracy. We finally analysed how preschool and primary school interact. We sampled 547 children who attended 97 German preschools. Latent growth curve analyses identified child and family factors related to age 3 numeracy and development to age 7: gender, migration background, socioeconomic status (SES), mother education, HLE. The effects of preschool on numeracy development persist until age 7 with notable effects from process quality. Strengthened efforts are needed to ensure high quality preschool education in Germany.     

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.     

The Home Literacy Environment Is a Correlate,but Perhaps Not a Cause,of Variations in Children’s Language and Literacy Development

The home literacy environment is a well-established predictor of children’s language and literacy development. We investigated whether formal, informal, and indirect measures of the home literacy environment predict children’s reading and language skills once maternal language abilities are taken into account. Data come from a longitudinal study of children at high risk of dyslexia (N = 251) followed from preschool years. Latent factors describing maternal language were significant predictors of storybook exposure but not of direct literacy instruction. Maternal language and phonological skills respectively predicted children’s language and reading/spelling skills. However, after accounting for variations in maternal language, storybook exposure was not a significant predictor of children’s outcomes. In contrast, direct literacy instruction remained a predictor of children’s reading/spelling skills. We argue that the relationship between early informal home literacy activities and children’s language and reading skills is largely accounted for by maternal skills and may reflect genetic influences.