Netherlands Journal of Psychology | Ge Covariance in a Longitudinal Twin Design 81 Accommodation of Genotype- Environment Covariance in a Longitudinal Twin Design

Authors: Johanna m. de Kort* , Conor V. Dolan*,** and Dorret I. boomsma** In the classical twin study, genetic and environmental influences on a phenotype are usually estimated under the assumption that genotype-environment covariance (Ge covariance) is absent. We explore possibilities to accommodate Ge covariance in longitudinal data using the genetic simplex model. First, the genetic simplex model is presented, accompanied by a brief summary of results found in cognitive developmental studies. Second, Ge covariance is specified via niche picking and sibling effects. Third, numerical and analytical identification is established, and the statistical power to detect Ge covariance is examined. In a simplex model comprising four time points, Ge covariance can be accommodated by introducing phenotype to environment cross-lagged pathways, either within or between twins. by using different parameter constraints within the genetic simplex, the extended models are numerically and analytically identified. The power to detect Ge covariance is relatively low and therefore large sample sizes are needed.