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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. (Contains 7 tables.)

As part of a longitudinal twin study of literacy and language, we conducted a behaviour-genetic analysis of orthographic learning, spelling and decoding in Grade 2 children (225 identical and 214 fraternal twin pairs) in the United States and Australia. Each variable showed significant genetic and unique environment influences. Multivariate analyses revealed very high genetic correlations among the variables, indicating that the same genes are involved in their aetiology. These genes are partly independent of those contributing to intelligence. A further analysis indicated that the covariation between decoding and orthographic learning is mediated by shared genes rather than by a direct causal path. The authors argue that a learning parameter, most directly assessed by orthographic learning in this study, underlies all three literacy variables. The results are also discussed in relation to Share's self-teaching hypothesis, which may require modification.

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 0.84 in Australia, 0.68 in the U.S., and 0.33 in Scandinavia. The effects of shared environment on kindergarten reading were estimated at 0.09 in Australia, 0.25 in the U.S., and 0.52 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 0.79 and 0.83 for reading and between 0.62 and 0.79 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. (Contains 7 tables.)

Grade 1 literacy skills of twin children in Australia (New South Wales) and the United States (Colorado) were explored in a genetically sensitive design (N = 319 pairs). Analyses indicated strong genetic influence on word and nonword identification, reading comprehension, and spelling. Rapid naming showed more modest, though reliable, genetic influence. Phonological awareness was subject to high nonshared environment and no reliable genetic effects, and individual measures of memory and learning were also less affected by genes than nonshared environment. Multivariate analyses showed that the same genes affected word identification, reading comprehension, and spelling. Country comparisons indicated that the patterns of genetic influence on reading and spelling in Grade 1 were similar, though for the U.S. but not the Australian children new genes came on stream in the move from kindergarten to Grade 1. We suggest that this is because the more intensive kindergarten literacy curriculum in New South Wales compared with Colorado, consistent with the mean differences between the two countries, means that more of the genes are "online" sooner in Australia because of accelerated overall reading development.

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.

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 grammarmorphology 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.

Individual differences in measures of prereading skills and in questionnaire measures of 4-5-year-old twins' print environments in Australia, Scandinavia, and the United States were explored with a behavioral-genetic design. Modest phenotypic correlations were found between environmental measures and the twins' print knowledge, general verbal ability, and phoneme awareness. Lower print knowledge in Scandinavian twins was related to country differences in preschool print environment. Latent-trait behavioral-genetic analyses indicated very strong shared-environment influences on individual differences in Print Knowledge. Genetic influence was also significant. Several other prereading skills varied in their environmental and genetic influence, including a significant contrast between Phonological Awareness and Print Knowledge. Rapid Naming also revealed very strong genetic influence, as did Verbal Memory. Stronger shared-environment influences were found for Vocabulary and Grammar/Morphology. Genetic and environmental correlations among latent traits for General Verbal Ability, Phonological Awareness, and Print Knowledge were high, but there were also significant independent genetic and environmental contributions to each skill. Practical implications include the need for substantial and sustained instructional support for children hampered by genetic constraints on early literacy development.

Preliminary results from data on 146 Australian, 284 American, and 70 Norwegian preschool twins indicate reliable genetic influences on phonological awareness and memory and learning. Vocabulary, grammar, and morphology showed significant shared environment and negligible genetic effects. A print knowledge composite showed genetic and shared environment influences. (Contains references.) (Author/CR)

Highlights: • First study comparing the etiology of variance in early reading growth between countries. • Reading data from twins in the U.S., Scandinavia, and Australia. • Balance of genetic and shared environmental influences varied at post-kindergarten. • High genetic influences on individual differences by the end of first grade, and on growth, in all countries. [ABSTRACT FROM AUTHOR]

Abstract: Early literacy and language skills of twin children in the USA, Australia, and Scandinavia were explored in a genetically sensitive design (maximum N =615 pairs). For this article, we report aspects of preschool and Grade 2 data. In Grade 2, there were strong genetic influences on word reading, reading comprehension, and spelling. Vocabulary was about equally affected by genes and shared environment. Multivariate analyses indicated substantial genetic overlap among the Grade 2 literacy variables. Longitudinal analyses showed that genetic factors evident at the preschool stage continued to affect literacy and vocabulary three years later in Grade 2, but there was also evidence of new genetic factors coming into play over the time interval, at least for literacy. Suggestions are made about the search for underlying biological and cognitive processes, and educational implications are explored. [Copyright &y& Elsevier]