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.     

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.     

Preschool twin study of the relation between attention-deficit/hyperactivity disorder and prereading skills

Numerous studies have examined the etiology of the association between reading difficulties and attention-deficit hyperactivity disorder (ADHD) in school-age children, but little is known about the relation between prereading skills and ADHD behaviors prior to the beginning of formal reading instruction. A population-based sample of 809 pairs of preschool twins completed an extensive battery of prereading measures, and the parent of each twin completed an ADHD rating scale. Phenotypic analyses revealed small but significant correlations between DSM-IV inattention ratings and six prereading composite measures, whereas hyperactivity–impulsivity symptoms were not independently associated with any of the prereading scores. Multivariate twin analyses indicated that virtually all of the phenotypic correlation between inattention and prereading performance is attributable to common genetic influences, consistent with results obtained in studies of older twins. Although additional research is needed to test alternative causal models in children younger than five years old, these results are most consistent with the hypothesis that reading difficulties and inattention symptoms are attributable to common genetic influences.     

Genetic and Environmental Influences on Temperament in Middle Childhood: Analyses of Teacher and Tester Ratings

Parent ratings of temperament in infancy and childhood yield evidence for genetic influence in twin studies but not in adoption studies. The present study used the sibling adoption design to investigate teacher and tester ratings of temperament in middle childhood. When each child was 7 years old, ratings on the Colorado Childhood Temperament Inventory were obtained from a teacher and tester for more than 50 pairs each of adoptive and nonadoptive siblings in the Colorado Adoption Project. Significant genetic influence emerged for both teacher and tester ratings of Activity, for tester ratings of Sociability, and for teacher ratings of Emotionality. Results obtained from bivariate genetic analysis suggest that the modest covariance between teacher and tester ratings of Activity is entirely mediated genetically. Except for teacher ratings of Attention Span, evidence of shared family environment was nonsignificant, despite the power of the sibling adoption design to detect it.     

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).     

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.     

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.