This study investigated young children's commitment to a joint goal by assessing whether peers in collaborative activities continue to collaborate until all received their rewards. Forty-eight 2.5- and 3.5-year-old children worked on an apparatus dyadically. One child got access to her reward early. For the partner to benefit as well, this child had to continue to collaborate even though there was no further reward available to her. The study found that 3.5-year-olds, but not 2.5-year-olds, eagerly assisted their unlucky partner. They did this less readily in a noncollaborative control condition. A second study confirmed that 2.5-year-old children understood the task structure. These results suggest that children begin to appreciate the normative dimensions of collaborative activities during the 3rd year of life.