Abstract
Triadic interactions between mother, pre-school age 1st-born, and 14 mo. old 2nd-born siblings were studied in 40 families as part of a longitudinal study using home observations. Interaction between mother and 2nd-born exerted an important influence on the behavior of the 1st-born; individual differences in the response of the 1st-born children were associated with differences in temperament of the 1st child, their reaction to the sibling birth, and with the sex of the sibling dyad. The age gap between siblings and the sex of sibling dyad influenced the time mother and 2nd-born spent in interaction and the quality of this interaction.