Genetic and environmental relations between vocabulary and reading skills were explored longitudinally from preschool through grades 2 and 4. At preschool there were strong sharedenvironment 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.Correspondence concerning this article should be addressed to Richard Olson, Department of Psychology, University of Colorado, Boulder CO 80303, Richard.Olson@Colorado.EDU.
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Author ManuscriptSci Stud Read. Author manuscript; available in PMC 2011 January 12. Much research has shown that individual differences in the development of vocabulary are significantly correlated with individual differences in reading, more strongly so with reading comprehension than with word recognition (Braze, Tabor, Shankweiler, & Mencel, 2007;Nation & Cocksey, 2009;Storch & Whitehurst, 2002). In addition, there is evidence for partial independence in the phenotypic contributions of oral language (including vocabulary) and decoding to reading comprehension as early as the second grade (Kendeou, van den Broek, White, & Lynch, 2009). A fundamental question about these individual differences is the relative influence from genetic and environmental factors on vocabulary, on different reading skills, and on their correlation. Are the same genetic and environmental influences at play for these skills, or are there significant differences in etiology between skills? The present study addresses this question longitudinally through the early stages of reading development by comparing the similarities of identical and fraternal twins beginning in preschool, and subsequently in a follow-up assessment near the end of the grade 2 when children are "learning to read," and then again at the end of grade 4 when children are "reading to learn" (Chall, 1983).The present analyses are based on data from the International Longitudinal Twin Study (ILTS) of early reading development. The ILTS includes samples of identical and same-sex fraternal twins from Australia, Colorado, and Scandinavia. The combined sample currently exceeds 2000 twins who have been tested in preschool. The twins were initially assessed on a broad range of pre-reading skills including letter name knowledge and vocabulary in the preschool year before admission to kindergarten, and before learning to read could interact with those skills. Subsequently the twins were tested on reading and related skills at the end of kindergarten, first, and second grade in all countries, and at the end of fourth grade in the U.S. sample. Vocabulary was assessed in preschool and again at the end of grades 2 and 4.In the present stud...