Young children are remarkably prosocial, but the mechanisms driving their prosociality are not well understood. Here, we propose that the experience of choice is critically tied to the expression of young children's altruistic behavior. Three- and 4-year-olds were asked to allocate resources to an individual in need by making a costly choice (allocating a resource they could have kept for themselves), a noncostly choice (allocating a resource that would otherwise be thrown away), or no choice (following instructions to allocate the resource). We measured subsequent prosociality by allowing children to then allocate new resources to a new individual. Although the majority of children shared with the first individual, children who were given costly alternatives shared more with the new individual. Results are discussed in terms of a prosocial-construal hypothesis, which suggests that children rationally infer their prosociality through the process of making difficult, autonomous choices.