Internalizing trajectories in young boys and girls: the whole is not a simple sum of its parts.

There is support for a differentiated model of early internalizing emotions and behaviors, yet researchers have not examined the course of multiple components of an internalizing domain across early childhood. In this paper we present growth models for the Internalizing domain of the Infant-Toddler Social and Emotional Assessment and its component scales (General Anxiety, Separation Distress, Depression/Withdrawal, and Inhibition to Novelty) in a sample of 510 one- to three-year-old children. For all children, Internalizing domain scores decreased over the study, although girls had significantly higher initial levels and boys had steeper declines. General Anxiety increased over the study period and, when modeled individually, girls evidenced higher initial levels and greater increases. For all children, Separation Distress and Inhibition to Novelty decreased significantly over time, while Depression/Withdrawal remained low without change. Findings from our parallel process model, in which all components were modeled simultaneously, revealed that initial levels of internalizing scales were closely associated while rates of change were less closely related. Sex differences in variability around initial levels and rates of change emerged on some scales. Findings suggest that, for one- to three-year-olds, examining scales of the internalizing domain separately rather than as a unitary construct reveals more meaningful developmental and gender variation.