SNOW, MARGARET ELLIS; JACKLIN, CAROL NAGY; and MACCOBY, ELEANOR E. Birth-Order Differences in Peer Sociability at Thirty-three Months. CHILD DEVELOPMENT, 1981, 52, 589-595. The relation of birth order to sociability with an unfamiliar peer was examined in 101 33month-old children. Only children were found to be the most sociable, firstborns the next most sociable, and later borns the least sociable. Children were also observed in a frustrating situation in which the peer was not present, and the frequency of assertive behavior was measured. It showed the same pattern of birth-order differences and was modestly related to peer sociability, suggesting that birth-order differences in sociability with a peer may be 1 manifestation of larger differences in a more general type of assertiveness. Family influences which may account for these birth-order differences are discussed.