Longitudinal twin study of early reading development in three countries: Preliminary results

Annals of Dyslexia, 2002 by Byrne, Brian, Delaland, Cara, Fielding-Barnsley, Ruth, Quain, Peter, Et al

We have initiated parallel longitudinal studies in Australia (Byrne, PI), the United States (Olson, PI), and Norway (Samuelsson, PI) of identical and fraternal twins who are being tested in preschool for prereading skills, and in kindergarten, first grade, and second grade for the development of early reading, spelling, and related cognitive skills. Comparisons of the similarities of identical and fraternal twins will reveal the relative influence of genetic, shared family environment, and nonshared environment on individual differences at and across different stages of development. Family and twin-specific environmental information is also being directly assessed through parent questionnaires and observations by testers. Most of the data collected so far have been from preschool twins (146 in Australia, 284 in the United States, and 70 in Norway). Preliminary analyses for the preschool cognitive measures showed reliable genetic influences on phonological awareness and several measures of memory and learning. In contrast, vocabulary, grammar, and morphology showed significant shared environment and negligible genetic effects. A print knowledge composite showed both genetic and shared environment influence.

OVERVIEW

It is now well established and widely accepted that individual differences in reading ability are, in part, heritable. Twin-based behavior-genetic studies with reading-disabled, school-aged individuals (DeFries & Alarcon, 1996; Gayan & Olson, 2001), with individuals in the normal range of reading ability (Davis, Knopik, Olson, Wadsworth, & DeFries, 2001), and with high ability individuals (Boada et al., in press) all indicate substantial genetic influence. The genetic effects extend to all aspects of reading including phonological decoding, orthographic knowledge, reading comprehension, and spelling (Olson, Forsberg, & Wise, 1994). Complementing the behavioral observations are studies at the molecular level with reading-disabled individuals and their parents and siblings, with susceptibility genes for dyslexia having been localized to chromosomes 2, 6, and 18 (Fisher et al., 2002; Gayan et al., 1999). Thus, there may be different genetic pathways to dyslexia across different individuals. We should also note that while the genetic influence for dyslexic group deficits as well as individual differences in reading and related skills are highly heritable, the relative importance of genetic and environmental factors may vary widely among different individuals with dyslexia and among different individuals across the normal range.

One aim of the present project is to gain a longitudinal perspective on genetic and environmental influences on literacy growth. All behavior-genetic research into literacy growth has so far been cross-sectional with school age children, and although there are indications of differential genetic effects as a function of age for component-skill deficits in reading and spelling (DeFries, Alarcon, & Olson, 1997), longitudinal studies offer the best prospects for identifying age-related trends in the amount and type of genetic and environmental influences. To this end, we elected to begin with preschool-aged twin children prior to their formal introduction to reading in the school system. As well as assessing relevant variables in these four-year-- olds as outlined below, we are following the children through the first several years of school to identify growth trends in a behavior-genetic framework. In this paper, we present preliminary data from the preschool phase of the project.

A second aim of this project is to examine genetic and environmental influences on cognitive, linguistic, and behavioral processes that are known to co-vary with reading ability and to do so in a design which may help disentangle causal relations among these processes and literacy levels themselves. Essentially, this means measuring these related processes prior to reading instruction, and particularly prior to reading failure. If processes considered as candidates for explaining variance in reading levels are assessed only after those levels are established, it is difficult to know whether the variables which are, in fact, related to reading skill are to be numbered among the causes or among the effects of reading level, or if indeed are related by means of a common process (Bradley & Bryant, 1983). By assessing candidate processes in young children prior to school entry, there is some assurance that variability in those processes is not due to variability in reading skill. If it further turns out that one or more of those processes shares genetic variance with subsequent reading skill, we may be closer to understanding how genes can influence this culturally determined ability, one that emerged only late in human history (Byrne, 1998; Man, 2000).

The logic of twin studies in estimating genetic influences on behavior is reasonably well known. Briefly, what is of most interest is the degree of correlation for monozygotic (MZ) twins compared to that of dizygotic (DZ) twins. MZ twins have all of their genes in common within pairs whereas DZ twins share, on average, half of their "segregating genes" (i.e., genes that vary across individuals). In contrast to this distinction between genetic similarities for the MZ and DZ twin types, it is assumed that both types have equally similar shared environments, such as the early home situation, although a child's genes may have an effect on his or her environment within the home, since parents respond differently to each child's unique personality and abilities (Segal, 2000). Given these facts and assumptions, any greater similarity between MZ twins compared to DZ twins on the variable of interest can be attributed to genes, and estimates of heritability can be obtained from within-pair covariances or correlations (Neale & Cardon, 1992). The effects of common (e.g., home) environment can also be detected in twin designs. If a behavioral process is largely under the influence of shared environment but not of genes, then the correlation between DZ twin pairs will be high, and similar to that between MZ twin pairs. Finally, nonshared environmental influences are reflected by the degree to which the correlation for MZ twins differs from 1, since shared environment and genetic influence account for the MZ twin correlation.