43. Cherny, S. S., Fulker, D. W., Emde, R. N., Robinson, J., Corley, R. P., Reznick, J. S., Plomin, R., & DeFries, J. C. (1994). A developmental-genetic analysis of continuity and change in the Bayley Mental Development Index from 14 to 20 months: The MacArthur Longitudinal Twin Study. Psychological Science, 5, 354-360.

More than 350 pairs of identical and same-sex fraternal twins were studied longitudinally at 14, 20, and 24 months of age. There was substantial genetic continuity of general cognitive ability from 14 to 24 months, but significant new genetic variation also appeared at 24 months. Shared family environmental influences were global across all three ages and not time-specific. Finally, nonshared environmental influences did not contribute to the observed continuity of general cognitive ability, only to change.