This paper presents research findings on differences in the social and cognitive behavior of first born and only children at 3, 12, and 24 months of age as observed at home and in a free play laboratory setting. The sample consisted of 21 only children (children who did not acquire a sibling for at least the first 48 months of life) and 35 first born children (children who acquired a sibling between the ages of 24 and 36 months). Results indicated that differences in only and first born children were related to three possible sources of variation: (1) the birth of a sibling for first born children; (2) child characteristics; and (3) parent characteristics. The birth of a sibling was noted to change the cognitive performance of first born children and was related to a drop in skill performance. This finding suggests that first born children are different from only children as a result of experiencing the entrance into the family of a new infant. Possible differences in the temperament characteristics of first born and only children were reflected in the tendency of only infants to cry more and smile less at 3 months of age than did first born infants. Finally, parents of only and first born children may be different: mothers of only borns were more involved in their infants, expressed the desire for less children, were older at the birth of their first child and experienced a more difficult pregnancy than mothers of first borns. (Author/MP)