Individual differences in changes in infants' interest in social signals in relation to developmental index

Detection of social signals, such as biological motion and social causality, is of basic importance in early infancy. There have also been some accounts that infants' visual preference or reaction to social signals change during development because of their changing understanding of such signals, and the detective abilities of primary social signals are related to later social development. In this study, we attempted to find different developmental patterns in individuals in terms of their visual preference for biological motion and socially causal movements at 4, 9, and 18 months and 4 and 9 months, respectively, using a cluster analysis. It was found that for both types of social signals, the infants who demonstrated an increased interest in social stimuli at 9 months scored higher on the developmental index than those who showed a decrease, suggesting a difference in the quality of understanding of social signals at 9 months.