Phenotypic g early in life -: On the etiology of general cognitive ability in a large population sample of twin children aged 2-4 years

We investigated the emergence of general cognitive ability (g) in early development and its genetic and environmental etiology. A representative population sample of 6963 pairs of twins was assessed on verbal and nonverbal measures at 2, 3, and 4 years using four indicators of cognitive ability derived from parent-administered tests and parental reports of children's abilities. Principal component factor analyses at each age clearly showed a single prominent g factor that accounted for between 50% and 55% of the total variance. These factor scores showed remarkable stability of .69 from ages 2-3 and .71 from ages 3-4. Twin models allowing for scalar sex differences suggested moderate but consistent additive genetic influences on phenotypic g (a(2) = .25-30),with most of the interindividual variance accounted for by shared environmental influences (c(2) = .61-.65). At age 2, a strong indication was found for a qualitative difference in etiology between the sexes, but otherwise results were similar for boys and girls. This study corroborates with a much larger sample three conclusions that have emerged from earlier research using standard tester-administered measures. First, phenotypic g is clearly evident early in life. Second, genetic influence is less in early childhood (about 20-30%) than in middle childhood (about 40%) and after adolescence (about 50%). Third, shared environmental influence is greater in childhood than after adolescence when its importance declines to negligible levels, although our very high estimate& of shared environmental influence (about 6ko) may include influences specific to twins and perhaps also to the measures used. (C) 2003 Elsevier Science Inc. All rights reserved.