To determine whether the presence of a fourth person affects triads in the same way that the presence of a third person affects dyads, data from two previous studies were reanalyzed. In the original studies a total of 60 infants 12 months of age were observed interacting with their parents in 4 social contexts: 1 parent present, 2 parents present, 1 parent and preschool-aged sibling present, and 2 parents and sibling present. Analyses revealed that (1) infants directed fewer behaviors to either parent when both were present than when only one was present; (2) infants directed fewer behaviors to either parent when the sibling was present than when she or he was absent; (3) a sibling's presence did not inhibit the infants' behavior as much as a parent's presence did; (4) the presence of the other parent or the sibling led parents to vocalize to their infants less; (5) the sibling's presence inhibited the parent's behavior less than the other parent's presence; and (6) the presence of a fourth person did not inhibit interaction significantly more than the presence of a third person. (Author/RH)