Exploring the genetic etiology of low general cognitive ability from 14 to 36 months.

The genetic and environmental etiology of low general cognitive ability (g) during infancy and early childhood has not previously been investigated. The current study examined the genetic etiology of low cognitive ability at 14, 20, 24, and 36 months with twins from the MacArthur Longitudinal Twin Study. Low g groups were formed from the lowest 10th percentile at each age. Univariate probandwise concordance rates and DeFries-Fulker (J. C. DeFries & D. W. Fulker, 1985, 1988) multiple regression techniques suggest genetic etiology in low general cognitive ability groups. The stability of low general cognitive ability over time also appears to be primarily due to genetic factors. Although replication is necessary, these results suggest that the genetic etiology of low g during infancy and early childhood is at least as great as the heritability of g in the unselected population.