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. Our International Longitudinal Twin Study (ILTS) of early reading development involves U.S., Australian, and Scandinavian samples of twins born between 1994 and 2000 in Colorado, the Sydney area, and in Sweden and Norway. The twins are assessed in preschool for prereading skills and their prereading environment. Their subsequent reading and spelling development is being assessed at the end of kindergarten and first and second grades (Byrne et al., 2002, 2005, 2006, this volume; Samuelsson et al., 2005; Willcutt et al., this volume). The overall aim of this cross-national and cross-language twin study is to identify genetic and Reading and Writing (2007) 20:51–75 Springer 2006 DOI 10.1007/s11145-006-9018-x