We examined the extent to which genes and the environment contributed to variation in and covariation among reading, spelling, grammar and punctuation, writing, and numeracy in Australian school children in Grades 3, 5, 7, and 9. Heritability was generally high: reading .58-.71 (excepting Grade 5 girls), spelling .68-.78; grammar and punctuation .52-.66, writing .39-.52, and numeracy .39-.79. Boys' performance varied more than girls in spelling and numeracy, and the common environment was a greater influence in girls than boys in Grade 3 numeracy and Grade 5 reading. 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. The covariation among the domains was largely mediated by genes. Results suggest substantial uniformity in the environmental factors influencing these academic domains.

Environmental factors play a key role in the development of Attention-Deficit/Hyperactivity Disorder (ADHD), but the long-term effects of these factors are still unclear. This study analyses data from 1024 monozygotic (identical) twins in Australia, the United States, and Scandinavia who were assessed for ADHD in Preschool, Kindergarten, Grade 1, and Grade 2. Differences within each twin pair were used as a direct measure of non-shared environmental effects. The Trait-State-Occasion (TSO) model developed by Cole et al. (Psychological Methods, 10, 3-20, 2005) was used to separate the non-shared environmental effects into stable factors, and transient factors that excluded measurement error. Stable factors explained, on average, 44 % and 39 % of the environmental variance in hyperactive-impulsive and inattentive symptoms, respectively. Transient effects explained the remaining 56 % and 60 % of variance. The proportion of stable variance was higher than expected based on previous research, suggesting promise for targeted interventions if future research identifies these stable risk factors.