Prosocial and normative behavior emerges in early childhood, but substantial changes in prosocial behavior in middle childhood may be due to it becoming integrated with children's understanding of what is normative. Here we show that information about what is normative begins influencing children's costly sharing in middle childhood in a sample of 6- to 11-year-old German children. Information about what is normative was most influential when indicating what was "right" (i.e., "The right thing is to choose this"). It was less influential when indicating what was prescribed by a rule (i.e., "There is a rule that says to choose this") or when it indicated what the majority of people do (i.e., "Most people choose this"). These findings support the idea that middle childhood is when social norms begin to shape children's costly sharing and provide insight into the psychological foundations of the relationship between norms and prosocial behavior.