70. Petrill, S.A., Saudino, K.J., Cherny, S.S., Emde, R.N., Fulker, D.W., Hewitt, J.K., & Plomin, R. (1998). Exploring the genetic and environmental etiology of high general cognitive ability in 14 to 36-month-old twins. Child Development, 69, 68-74.

Although numerous theories have attempted to explain the origins of high general cognitive ability (g), the genetic and environmental etiology of high g during infancy and early childhood has not previously been investigated. We report results of a twin study of high cognitive ability at 14, 20, 24, and 36 months using twins from more than 600 children. High g groups were formed from the 90th percentile and above at each age, with IQ equivalent means at or above 126 across the ages. Results suggest increasing genetic influence and increasing stability from 14 to 36 months using multiple regression analyses. However, genetic influences are substantial when examining individuals who possess high g scores averaged across all four ages. These results suggest that, although high cognitive ability may be genetically influenced in early childhood, these influences differ in magnitude from 14 to 36 months.