Changes in cooperation and self-other differentiation during the second year.

Cooperation in peer interaction emerges during the second half of the second year. A consideration of the skills and knowledge entailed in these early forms of cooperation suggests that young children's emerging ability to differentiate self from other as causal agents may relate to their ability to coordinate behavior with age mates toward a common goal. Children at 12, 18, 24, and 30 months were observed in same-age, same-sex dyads (8 dyads per age) while attempting to solve a simple cooperation problem. They were also individually administered an elicited imitation task used to index decentration, or self-other differentiation. No 12-month-old dyad could cooperate, 18-month-olds did so infrequently and apparently accidentally, whereas 24- and 30-month-olds were able to coordinate behavior with one another quickly and effectively. Children who were better able to accommodate their behavior to one another during cooperation also represented the agency of others at a more advanced, decentered level.