A longitudinal twin study of stability of components from Bayley's Infant Behavior Record.

Factor scores from Bayley's Infant Behavior Record, obtained from about 300-400 infant twins at 6, 12, 18, and 24 months, were selected to represent 3 aspects of infant behavior: task orientation, test affect-extraversion, and activity. Age-to-age correlations for the scores yielded a simplex pattern of low to moderate order. Further age-to-age analyses by twin pairs generally showed that the identical twins, as compared with same-sex fraternal twins, generated more within-pair similarities for profiles of the factor scores across 2 overlapping age ranges: 6-18 months, and 12-24 months. Overall, the findings indicated that there is reordering of individual differences for aspects of infant behavior from one age to the next, but the sequence of reordering is somewhat structured in accord with genotypic similarity. The longitudinal findings are also discussed within the context of change and continuity of the individuality of infant temperament.