Mechanisms of developmental change in genetic and environmental influences on intelligence

Abstract

We attempted to replicate previous findings of rising heritability and declining shared environmental influence on intelligence across development using the Twins Early Development Study—a large, longitudinal sample of British twins. Two longitudinal twin models (a Cholesky and Simplex model) were fitted to 9 waves of cognitive test data for 10,535 twin pairs. Heritability rose significantly from 24% at age 2 to 48-52% at age 16. Concurrently, shared environmental influence declined significantly from 65% to 9-13%. Moreover, nonshared environmental influences also increased significantly, rising from 11% to 39-40%. In early childhood, developmental gains in heritability were explained by the emergence of new genetic influences. In later childhood and adolescence, gains in heritability were explained by the continued emergence of new genetic influences but also the amplification of pre-existing genetic influences. This evidence for genetic amplification suggests that active gene-environment correlation is partly responsible for gains in heritability later in development. Shared environmental influence declined across development because pre-existing environmental influences decayed between waves faster than new environmental influences accumulated. Lastly, the rise in nonshared environmental influence was principally explained by age-specific environmental effects that increased in magnitude across development.