The data presented in this brief report provided a systematic test of the genetic and environmental sources of variance in reading skills among children in kindergarten and first grade. Multiple measures of reading-related outcomes were employed; in the case of g, phonological awareness, and rapid automatized naming, multiple measures were employed to form composite measures. In contrast to previous studies of older children, shared environmental influences were significant and substantial, accounting for one third to one half of the variance for g, letter identification, phonological decoding, and phonological awareness. In contrast, familial resemblance for rapid automatized naming was influenced almost completely by genetic variance.
In particular, the c2 estimates derived from the twin-only models (see ) were mirrored by the adoptive sibling intraclass correlations (see ). Although they provide a direct test of the shared environment, adoptive siblings have many potential differences that might attenuate the magnitude of their correlations. The 30 adoptive sibling pairs participating in this study were generally from the same countries, but children from these 30 families were born in many different countries, each with important differences in age of placement, quality of the preadoptive environment, and exposure to English language sounds. Despite these differences, adoptive intraclass correlations were significantly different from zero for phonological awareness and word attack, and nonsignificant trends were found for letter identification and word identification. More important, with the exception of g, the adoptive sibling intra-class correlations (our only direct test of shared environmental variance) were very similar to the estimates of shared environmental variance derived from the modeling of the twin data (see ). Taken together, these results suggested significant shared environmental influences that were generally consistent across twin and adoption designs.
Thus, similar to Byrne et al. (2002)
, the evidence for significant shared environmental influences in the current study depended on the type of reading measure—with content-based reading measures (e.g., letter identification) more likely to show shared environmental influence than process-based measures of reading (e.g., rapid automatized naming). However, unlike Byrne et al. (2002)
, we found significant and substantial shared environmental influences for phonological awareness and decoding. These findings suggest that c2
accounts for 30% to 40% of the individual differences in phonological skills, whereas Byrne et al. (2002)
found that c2
accounted for 15% to 25%. These differences may be explained by sampling differences between the two studies; as mentioned earlier, Byrne et al.’s sample was composed of children from the United States, Australia, and Scandinavia, whereas the samples for the current study were composed of twin pairs recruited in the midwestern United States and domestically and internationally adopted children living in the Pacific Northwest and New England. However, it is important to note that the point estimates of shared environmental influence in Byrne et al.’s study, though nonsignificant, were not zero. Thus, the variability in results may be due to normal variance in the sampling distribution of the population of children who are learning to read.
More generally, as noted in the results section, the behavioral genetic method possesses much greater power to detect whether h2, c2, and e2 estimates are significantly different from zero—as opposed to detecting whether these estimates are different from each other. This was evidenced in the confidence intervals presented in the current study, which were different from zero in most cases but generally overlapped with one another.
Nevertheless, the current study suggests a pattern of statistically significant shared environmental influences that is largely consistent with the results of Byrne et al. (2002)
and is different from the findings of studies examining older children (e.g., Gayan & Olson, 2003
). Although not explicitly tested, the data in the current study and in Byrne et al. (2002)
are consistent with a developmental shift in the etiology of individual differences in reading skills in the early school years, particularly for content-based reading skills. When examining other cognitive skills, such as general cognitive ability, researchers have found a consistent trend where genetic and shared environmental influences are both moderate (h2
= .30–.40) in younger children. In middle childhood, h2
begins to get larger at the expense of c2
. By adolescence, h2
approaches .60 to .80, and shared environmental variance approaches zero (see McGue, Bouchard, Iacono, & Lykken, 1993
). This trend is found whether looking at different children at different ages (Petrill, 2003
) or at the same children as they age (Plomin, Fulker, Corley, & DeFries, 1997
Research examining the development of reading skills has emphasized the distinction between young children who are “learning to read,” as opposed to older children who are “reading to learn.” For example, Chall (1983)
argued that young children who are learning to read are primarily tasked with learning to read words that already are present in their oral vocabulary. The main requirements of successfully learning to read at this stage are phonological awareness, orthography, and visual-analytic ability (see Dale & Crain-Thoreson, 1999
). As reading skills mature, children are able to use reading to learn new words and to integrate these words into their developing semantic knowledge. Therefore, it is sensible that shared environmental influences would be greater for outcomes that are more likely to be influenced by direct instruction in the home, such as expressive vocabulary or print knowledge, as opposed to outcomes such as rapid automatized naming.
Moreover, research examining the environmental predictors of early literacy has suggested that reading-related knowledge and skills that children acquire in the home environment are associated with early reading success (McCardle, Scarborough, & Catts, 2001
), but that the indices of the home environment that are important in young readers are no longer influential among older readers (Scarborough & Dobrich, 1994
). It is also clear that phonological and print-letter skills become less important to later reading achievement, in favor of comprehension skills (e.g., Curtis, 1980
). Thus, we should not be surprised that the shared environment is significant in these measures among children learning to read but is modest or negligible among older readers.
Taken together, these findings suggest that individual differences in early reading may be a function both of genetic differences in early readers and of familial differences in parent-driven, shared environmental influences, such as book sharing, parental verbal skills, and parental educational attitudes. As children learn to read, genetically mediated, child-driven influences may emerge that affect the probability of coming into contact with experiences associated with positive or negative reading outcomes. Some examples of these experiences may be children’s enjoyment of reading, children’s desire to read on their own, and teachers’ responses to children based on their reading skills. In previous studies and in this study, we have found that the shared environment is important in early reading when examining sibling correlation data, parent-offspring data, and the links between measured environments and child reading outcomes. As longitudinal data in the Western Reserve Reading Project become available, we will be able to answer two additional questions: To what extent do genes and environments influence the stability or instability of reading skills over time? and, to what extent do measured environmental influences on early reading shift from a shared environmental to a genetic etiology as children learn to read and as the environments associated with reading become more a function of their own reading skills?