Becoming a social partner with peers: cooperation and social understanding in one- and two-year-olds.

One- and two-year-old peer dyads were presented with a simple cooperative task. Age differences were found in amount of coordinated activity, monitoring the peer's activity and location in relation to the goal, and attempting to achieve the goal when the peer was (or was not) available as a partner. One-year-olds' coordinated actions appeared more coincidental than cooperative whereas older children appeared to be more actively cooperating toward a shared goal. Differences in coordinated activity with peers were associated with differences in attention sharing with an adult and with language about self and other. The ability to cooperate with peers, becoming a true social partner, develops over the 2nd and 3rd years of life in concert with growing social understanding.