Abstract
The transition to siblinghood represents a period of adjustment for parents and firstborn children, influencing child-parent attachment relationships and children's emotional development. This study examined the longitudinal bidirectional relations between firstborn children's emotion understanding and their attachment security to their mothers and fathers during this transition. Firstborn children (N = 230, 55% girls/45% boys, 86% White, 5% Black, 3% Asian, 4% Latinx, and M(age) = 29.75 months) completed the Emotion Understanding in Early Childhood scale, and both mothers and fathers completed the Attachment Q-Set at three time points: prenatal, 4, and 12 months after the birth of an infant sibling. Structural equation modeling revealed that firstborn children's attachment security to mothers was associated with their emotion understanding before the sibling's birth. However, we found no evidence of bidirectional relations between children's emotion understanding and their attachment security to either fathers or mothers over time. These findings suggest that the older siblings' emotion understanding in the year following the birth may be due to other family-level dynamics during the transition besides the security of mother-child and father-child attachment, including early interactions with their baby sibling. Future work would benefit from including early sibling interactions as well as the broader family context when accounting for growth in the older sibling's emotion understanding after the birth of a new sibling. (PsycInfo Database Record (c) 2026 APA, all rights reserved).