Spinath, F. M., Ronald, A., Harlaar, N., Price, T. & Plomin, R. (im Druck). Phenotypic 'g' early in life: On the etiology of general cognitive ability in a large population sample of twin children aged 2 to 4 years. Intelligence.
We investigated the emergence of general cognitive ability (g) in early development and its genetic and environmental etiology. A representative population sample of 6,963 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 0.69 from ages 2 to 3 and 0.71 from ages 3 to 4. Twin models allowing for scalar sex differences suggested moderate but consistent additive genetic influences on phenotypic g (a2=0.25 - 0.30) with most of the interindividual variance accounted for by shared environmental influences (c2=0.61 - 0.65). 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 estimates of shared environmental influence (about 60%) may include
influences specific to twins and perhaps also to the measures used.
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