Development genetic analysis of general cognitive ability from 1 to 12 years in a sample of adoptees, biological siblings, and twins

We report a longitudinal genetic analysis that combines twins and adoptive and nonadoptive siblings to investigate continuity and change in the etiology of general cognitive ability from infancy through the transition to adolescence. Research in childhood suggests that heritability increases and shared environmental influence decreases, that genetic factors contribute to change as well as continuity, that shared environment contributes entirely to continuity, and that nonshared environment contributes entirely to change. Twins from the Longitudinal Twin Study (LTS; 224 MZ pairs, 189 same-sex DZ pairs at 1 year) and adoptive (genetically unrelated) and nonadoptive (biological) siblings from the Colorado Adoption Project (CAP; 107 and 87 pairs, respectively, at 1 year) were assessed again at 2, 3, 4, 7, 9, and 10 years, and nontwin siblings were also assessed at 12 years. Longitudinal model fitting supported the above hypotheses derived from research in childhood with two exceptions. Nonshared environmental influences contribute to continuity as well as change in middle childhood. The most striking exception is that during the transition to adolescence, genetic factors no longer contribute to change, just to continuity. (C) 2003 Elsevier Science Inc. All rights reserved.