Using Genetically Informative Family Data to Examine the Early Development of Complex Traits

This project triangulated evidence from three genetically sensitive family designs-classical twin studies, a parent-offspring adoption study, and genomic analysis of mother-father-child trios-to explore the roles of genotypes, environments, and interplay between the two, in the development of complex traits. It used these designs to address two unresolved questions: (1) What are the earliest manifestations of genetic and environmental influences on psychological, developmental, and academic traits? (2) Are the effects of genetic influences on academic outcomes environmentally mediated via evoked differences in the early caregiving environment? These questions were explored in four empirical chapters: The first systematically reviewed and meta-analysed the large twin study literature on genetic and shared and nonshared environmental influences on individual differences in psychological traits and developmental milestones in infancy. The second examined the earliest manifestations of genetic influences on academic abilities, demonstrating that variation in language may be an important early manifestation of genetic influences on later academic performance. The third and fourth chapters examined, using two different genetically informative methods (the adoption design and genomic analysis of mother-father-child trios), whether the effects of genetic influences on academic performance were mediated via evoked differences in the early caregiving environment. One, but not the other, found evidence that parents may adjust their parenting based on their children's education-associated genetic predispositions. Neither found evidence that parenting mediated genetic effects on academic performance, but they both replicated the finding that language mediates genetic effects on academic performance, suggesting that early language may be an early marker of genetic effects on later academic outcomes. Overall, this project provides new information about the earliest manifestations of genetic and environmental effects, and interplay between the two, on individual differences in complex traits. In doing so it demonstrates that genetically sensitive family-based research can be leveraged not only to further knowledge of genetic influences but also to better understand environmental pathways in complex trait development