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 (a2=.25–.30) with most of the interindividual variance accounted for by shared environmental influences (c2=.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 estimates of shared environmental influence (about 60%) may include influences specific to twins and perhaps also to the measures used.