GENETIC INFLUENCE ON TESTER-RATED INFANT TEMPERAMENT AS ASSESSED BY BAYLEY INFANT BEHAVIOR RECORD - NONADOPTIVE AND ADOPTIVE SIBLINGS AND TWINS

Results are reported for the 1st sibling adoption study of temperament in infancy, using tester ratings on the Infant Behavior Record (IBR) at 1 and 2 years for a sample of 95 pairs of nonadoptive siblings and 80 pairs of adoptive siblings. Twin data reported by Matheny (1980) for 85 identical (MZ) and 50 fraternal (DZ) twin pairs on the same measure at the same ages were combined with the sibling adoption data in maximum-likelihood model-fitting analyses for 3 IBR factors (affect-extraversion, activity, and task orientation). Both the sibling adoption and twin data yielded evidence for significant genetic influence for the 3 IBR factors at 12 and 24 months: Heritability estimates ranged from 35% to 57%. Shared environment contributed little to the total variance for any of the 3 traits.