Prosociality emerges early in ontogeny, but the mechanisms driving its early-emergence are not well understood. We propose that the experience of choice is tied to the expression of children’s prosocial behavior. In Experiment 1, preschoolers shared with a puppet by either making a Costly Choice (giving a resource they could have kept for themselves), Non-Costly Choice (giving a resource that would otherwise be thrown away), or No Choice. Subsequent prosociality was measured by allowing children to share with a new puppet. While most children shared initially, children who were given costly choices shared more with the new puppet. Experiment 2 replicated this result using a different manipulation for Costly vs. Non-Costly choices. Experiment 3 found that preschoolers were more likely to infer that actions are intentional when they are costly. Results suggest a prosocial construal hypothesis: that children rationally infer their prosociality through making difficult, autonomous choices.