Genetic and environmental influences on cognition during childhood

The basic methodology of behavior genetics is described in the context of the twin and sibling/adoption design. Results of applying this model to cross-sectional data on cognitive development throughout the lifespan are presented. Results from a large longitudinal twin and adoption study of general intelligence are presented to illustrate the use of the basic behavior genetic model in studying the causes of continuity and change in development. A special model is presented for the study of abnormal behavior occurring at the extremes of the distribution and an adaptation of the model is discussed for the study of genetic linkage or the mapping of quantitative trait loci (QTL). Results of applying this model to a sample of twins and kindred siblings indicated the presence of a major QTL for dyslexia on chromosome 6. The purpose of the paper is to illustrate the scope of behavior genetic methodology in studying individual differences in behavior in a biosocial context.