<p>Each year, Australian students in Grades 3, 5, 7, and 9 sit nationwide tests in literacy and numeracy. These tests inform government, principals, and parents about student, school, and state performance in five domains: reading, spelling, grammar and punctuation, writing, and numeracy. As such, the results of these tests are of wide interest for diverse reasons depending on the stakeholder in question. In this thesis I examine the influence of genes and the environment on individual differences in performance on these tests. Using longitudinal data collected from a large sample of Australian twins and their siblings.</p>
<p>Initially, as a test of validity, I compared the performance of large-scale reading tests against three literacy tests in comprehension, word reading and vocabulary individually administered to twins in Grade 3. The individually administered tests accounted for a substantial amount of the variance in the large-scale reading tests. Additionally, they were preferentially related, both genetically and environmentally, to large-scale reading tests compared to large-scale numeracy tests, confirming that large-scale school reading tests measure, at least in part, the literacy skills tapped by individual tests considered “gold-standard” in testing.</p>
<p>In the second paper, I examined the extent to which genes and the environment contributed to variation in and covariation among the five domains in each grade. Averaged across domains and grade, genetic factors explained 60%, shared environment 10%, and unique environment 30% of the variation. Independent pathway models showed similar genetic and environmental structures at each grade with approximately one third to one half of the variation in each domain due to genes that influenced all domains.</p>
<p>In the third paper, I explored the genetic and environmental influences on stability and growth in each of the domains. Stability in performance was primary due to genes. For growth, reading followed a compensatory growth pattern, and variation in growth was due to the genes that also influenced differences in performance at initial testing. By contrast, growth in numeracy was principally influenced by unique environmental factors. These results suggest individual differences in growth of reading are primarily due to a genetically influenced developmental delay in the acquisition of necessary skills, while for numeracy, differences are due to environmental influences, such as different teachers or interests.</p>
<p>In the fourth paper, I tested if family or school SES moderated heritability of performance. Genetic influence was substantial and stable across all levels of family and school SES, with some evidence of a stronger influence of the shared environment when SES was lower, particularly for Grade 3 literacy. A final chapter presents a discussion summarising the principal findings, their implications, and their limitations.</p>