Genetic and environmental influences on the growth of early reading skills

Over the past 25 years, research has led to important progress in the understanding of early reading development, establishing the importance of vocabulary, letter–sound correspondence, phonological processing, fluency, and language comprehension (see Whitehurst & Lonigan, 1998; National Reading Panel, 2000). Studies have also shown that there is considerable variability not only for initial reading level when children begin formalized instruction, but also in the rate of subsequent growth (Parrila, Aunola, Leskinen, Nurmi, & Kirby, 2005). Researchers have gained insight into this variability by examining indices of the family and school that predict individual differences in initial reading as well as growth in performance. Children from higher-SES households tend to have higher initial reading scores and show faster rates of growth compared with children from lower-SES households (e.g., Aikens & Barbarin, 2008; Cheadle, 2008). Additionally, initial rates of reading performance and rates of subsequent growth are higher when parents show high levels of educational involvement (Cheadle, 2008) and provide rich home literacy experiences (Aikens & Barbarin, 2008). School factors such as the type and quality of teacher instruction have also shown significant relations with student reading outcomes (Connor et al., 2009; Foorman et al., 2006; National Reading Panel, 2000) and have also been implicated as significant contributors to growth in student reading performance during the school year (Foorman, Francis, Fletcher, Schatschneider, & Mehta, 1998; Nye, Konstantopoulos, & Hedges, 2004). At the same time, quantitative genetic studies have established the importance of both genetic and environmental influences on overall reading ability and disability (e.g., DeFries, Fulker, & LaBuda, 1987; Harlaar, Dale, & Plomin, 2007; Petrill, Deater-Deckard, Thompson, DeThorne, & Schatschneider, 2006a; Samuelsson et al., 2008; Stevenson, Graham, Fredman, & McLoughlin, 1987), as well as on the skills that support reading ability, including vocabulary, print knowledge, phoneme awareness/decoding, spelling, and orthographic coding (e.g., Bates et al., 2004; Byrne et al., 2008; Friend, DeFries, Wadsworth, & Olson, 2007; Gayan, & Olson, 2003; Petrill, Deater-Deckard, Thompson, DeThorne, & Schatschneider, 2006b). To date, behavioral genetic studies have focused on two issues related to reading development. First, studies have examined how the relative magnitude of genetic and environmental effects varies as a function of child age. For example, genetic effects tend to be consistently important at all ages for fluency, and tend to become increasingly more important with age for phoneme awareness (Byrne et al., 2005; Petrill et al., 2007). At the same time, environmental influences appear to be consistently important across age for letter knowledge and word knowledge (Petrill et al., 2007) and may become more important with age for spelling (Friend et al., 2007). Second, behavioral genetic research has also focused on the stability of reading outcomes; in particular whether the same genetic and environmental factors remain consistent over time (e.g. Byrne et al., 2007; Harlaar et al., 2007; Betjemann et al., 2008; Wadsworth, DeFries, Olson, & Willcutt, 2007; Petrill et al., 2007). These studies suggest that the stability and instability in reading skills is substantially influenced by genetic factors. However, these studies also provide evidence for environmental overlap across measurement occasions as well as the emergence of new environmental factors as reading skills develop. However, behavioral genetic studies to date have not examined growth in reading performance – in particular, how genetics and environment impact individual differences in how quickly or slowly children’s reading and related skills improve over time. This is unfortunate given that much of the larger contemporary reading literature, particularly the intervention literature, has focused on growth as an outcome measure. Genetic and environmental factors may not just contribute to the variance of reading skills at a particular age or to the stability of individual differences of reading skills at different ages, but also to the rate at which children make reading gains. Another important issue is the extent to which genetic and environmental influences on the rate of reading growth are independent from genetic and environmental influences on initial performance. This is especially important in early reading where children come to formalized instruction with genetic variation related to reading and varying levels of preschool environmental supports for literacy; and are then educated in settings of varying instructional quality. Examining individual differences in growth in a genetically sensitive design may shed new light on children’s reading development, in particular, how genetic and environmental influences on subsequent growth are related to or independent from those genetic and environmental factors impacting initial performance. Thus, the purpose of the current study was to conduct the first behavioral genetic analysis on the growth of reading skills. We examined 6 reading-related skills: word identification, letter identification, pseudoword decoding, expressive vocabulary, phoneme awareness, and rapid naming. We determined the extent to which genetic and environmental influences contributed to variance in: (1) initial levels of reading, (2) rates of growth, and (3) the association between initial level and rates of growth, including the degree to which the rate of growth was correlated with or independent from initial levels of reading.