Abstract: Firstborn children’s reactions to mother-infant and father-infant interaction after a sibling’s birth were examined in an investigation of 224 families. Triadic observations of parent-infant-sibling interaction were conducted at 1 month after the birth. Parents reported on children’s problem behaviors at 1 and 4 months after the birth and completed the Attachment Q-sort before the birth. Latent profile analysis (LPA) identified 4 latent classes (behavioral profiles) for mother-infant and father-infant interactions: regulated-exploration, disruptive-dysregulated, approach-avoidant, and anxious-clingy. A fifth class, attention-seeking, was found with fathers. The regulated-exploration class was the normative pattern (60%), with few children in the disruptive class (2.7%). Approach-avoidant children had more behavior problems at 4 months than any other class, with the exception of the disruptive children, who were higher on aggression and attention problems. Before the birth, anxious-clingy children had less secure attachments to their fathers than approach avoidant children but more secure attachments to their mothers. Results underscore individual differences in firstborns’ behavioral responses to parent-infant interaction and the importance of a person-centered approach for understanding children’s jealousy. (PsycINFO Database Record (c) 2014 APA, all rights reserved).