Bayley's infant behavior record: behavioral components and twin analyses.

Factor analysis of 25 rating scales from Bayley's infant behavior record were performed for a sample of about 300-400 infant twins tested at 1 or more of the following ages: 3, 6, 9, 12, 18, and 24 months. The analysis provided 5 major factors and 2 minor factors that were considerably consistent at all ages. As defined by the scales with the strongest loadings, the major factors were denoted as task orientation, affect-extraversion, activity, auditory-visual awareness, and motor coordination, and the minor factors were denoted as kinds of mouthing behaviors. Further analyses by twin pairs generally provided higher concordance for identical pairs than for same-sex fraternal pairs, and the differences in concordance became more evident with an increase in age. Among the factors, task orientation showed the most persistent differences between identical and fraternal pairs. Profile analyses for all factors extracted at each age indicated that the profiles within identical pairs were more similar than profiles within fraternal pairs. Overall, the findings delineate some of the dimensions of temperament observed during testing and suggest that there are genetic influences on several behavioral characteristics and the organization of those characteristics during infancy.