Coparenting moderates the association between firstborn children's temperament and problem behavior across the transition to siblinghood.

The contribution of individual (i.e., negative reactivity) and environmental (i.e., coparenting) characteristics in predicting firstborns’ adjustment after a sibling's birth were examined. Mothers, fathers, and firstborn children from 241 families participated in a family freeplay to assess coparenting interactions before the birth of the second child and parents completed questionnaires on children's temperamental characteristics and behavior problems. Children's externalizing problems significantly increased from pre- to post-birth. Children, on average, did not display more internalizing problems following the infant sibling's birth; however, children high in negative reactivity were more sensitive to undermining coparenting behavior and displayed greater internalizing behaviors across the transition to siblinghood. Negatively reactive children also displayed increases in externalizing behavior across the transition to siblinghood when parents showed high levels of undermining coparenting and low levels of supportive coparenting. Supportive coparenting appeared to be a protective factor in the face of this transition for negatively reactive children in families where parents engaged in high levels of undermining coparenting. Findings suggest that both individual and environmental factors play an important role in firstborns’ adjustment to an infant sibling's birth. Parents of temperamentally sensitive children may benefit from participating in workshops geared towards improving coparenting partnerships prior to the birth of the second child.