This study aimed at investigating developmental changes in 3-, 4-, and 5-year-old children’s sharing behavior and their expectations of others’ sharing. Children were administered two tasks. In the Self task, they could distribute valuable items between themselves and a friend or a disliked peer; in the Other task, they were asked to predict how another agent would distribute valuable items between himself and a friend or a disliked peer. Additionally, whether sharing was costly for the agents or not was manipulated. Three results:

1. Basic prosocial orientation: Children of all age groups behaved more prosocially and expected more prosocial behavior from another protagonist when the choice bore no cost. This is kind of an obvious result in view of the existing literature. Previous studies have shown that children act prosocially from early on and distribute resources equally between others. Children also have a corresponding expectation that others will behave prosocially. Even 2-year-olds show a sensitivity for equal distributions in a looking-time task. By 3 years, children showed a general disposition to expect that someone will share with others; at this age, children possess an undifferentiated expectation that humans behave prosocially toward each other.
2. Recipient-dependent sharing: However, 4- and 5-year-old children, but not 3-year-old children, differentiated between a friend and a disliked peer as potential recipients in the sharing and the sharing expectation tasks. Thus, the study found developmental changes, with 3-year-old children not differentiating between different recipients (the 3-year-old children decided to act prosocially in the majority of trials) and 4- and 5-year-old children showing a clear differentiation. The 4- and 5-year-old children expected someone to share more with a friend than with a disliked peer, indicating specific expectations of how the relationship between an agent and another person affects the probability of showing prosocial behavior. This shows that the undifferentiated expectation that people generally share with others becomes differentiated in the course of the preschool period.
3. Relationship between first-person behavior and third-person expectations: The same developmental trend was found for children’s own sharing and their expectations of other people’s sharing behavior, suggesting that both show a parallel developmental progression on a group level. Moreover, at 5 years of age, but not at 3 or 4 years, sharing behavior and sharing expectations were on a personal level closely related to each other. In other terms, a within-subject relation was found between 5-year-old children’s own sharing behavior and their sharing expectations. In conclusion, the relation between sharing behavior and sharing expectations emerges strongly at 5 years of age.