59. Saudino, K. J., Plomin, R., & DeFries, J. C. (1996). Tester-rated temperament at 14, 20, and 24 months: Environmental change and genetic continuity. British Journal of Psychology, 14, 129-144.

Sources of continuity and change in temperament was examined using Infant Behavior Record (IBR) data from 301 twin pairs at 14, 20, and 24 months of age. Cross-age correlations for the three IBR factors (activity, task orientation, and affect-extroversion) were moderately low, suggesting that temperament in the second year of life is marked by change rather than continuity. At each age, there was significant genetic influence on each temperament dimension, which did not change from age to age. Shared environmental influences were nonsignificant at all ages; there were substantial nonshared environmental influences unique to each age. Change, which predominates over continuity in tester-rated temperament in the second year of life, can be attributed to environmental factors. Continuity appears to be due to genetic factors.