Tester-rated temperament at 14, 20 and 24 months: Environmental change and genetic continuity

The present paper examines genetic and environmental sources of continuity and change in temperament in a sample of 301 twin pairs participating in the MacArthur Longitudinal Twin Study. At 14, 20 and 24 months of age, temperament was assessed independently for each twin using tester ratings on the Infant Behavior Record (IBR; Bayley, 1969). Cross-age correlations for the three IBR factors (Activity, Task Orientation and Affect-Extraversion) 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. Moreover, the magnitude of heritability was similar across age, suggesting that heritability does not change dramatically from 14 to 24 months of age. The results of model-fitting analyses of continuity and change were similar for each IBR factor: no new genetic effects emerged at 20 or 24 months; shared environmental influences were non-significant at all ages; and there were substantial non-shared environmental influences unique to each age. Thus, change, which predominates over continuity in tester-rated temperament during the second year of life, can be attributed to environmental factors. Continuity appears to be due to genetic factors.