Longitudinal twin study of early literacy development: Preschool and kindergarten phases

We conducted behavior-genetic analyses of kindergarten reading, spelling, phonological awareness, rapid naming, and spoken sentence processing in 172 pairs of monozygotic and 153 pairs of same-sex dizygotic twin kindergarten children sampled in the United States and Australia. We also modeled progress from preschool to kindergarten in literacy-related variables, with larger numbers of twins contributing to the preschool phase. Reading, phonological awareness, and rapid naming at kindergarten. showed substantial effects of genes and modest,effects of shared environment; spelling was influenced by genes and environment equally; and sentence processing was affected primarily by shared environment. Longitudinal analyses indicated that the same genes affect phonological awareness in preschool and kindergarten but that a new genetic factor comes into play in rapid naming as letters and digits are introduced in kindergarten. At preschool, print knowledge and phonological awareness share one source of genetic influence, which in turn affects reading and spelling in kindergarten. Phonological awareness is subject to a second genetic factor, but only the one it shares with print also influences kindergarten reading and spelling. In contrast to the genetic effects, a single source of shared environment affects preschool print knowledge and phonological awareness and kindergarten reading. The results are discussed in the context of theoretical and practical issues in literacy development.