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The Structure of Developmental Variation in Early Childhood

Do children’s abilities develop in tandem or on their own separate timetables? Piaget proposed that development proceeded globally through stages; more recent theories view development as more modular with diﬀerent abilities developing independently and on diﬀerent time-scales. The developmental diﬀerentiation hypothesis suggests that the structure of a child’s development is unitary early in infancy but becomes more complex with age. Despite an abundance of theoretical interest in this question, there is little empirical work on the macrostructure of developmental changes in early childhood. We investigate this structure using two large datasets of parent-reported developmental milestones. Applying item response theory models, we ﬁnd that variation in development across infancy and early childhood is multidimensional. Consistent with the diﬀerentiation hypothesis, diﬀerences among older children are better described by higher-dimensional models. In addition, in longitudinal data, we ﬁnd that, within-person changes in underlying abilities are highly coupled early in life but their coupling decreases by age 12 months. Our work provides a model-based method for linking holistic descriptions of early development to basic theoretical questions about the nature of change in childhood.