Children cooperate more successfully with non-kin than with siblings.

Cooperation plays a key role in advanced societies with human cooperation among kin being more prominent than cooperation among non-kin. However, little is known about the developmental roots of kin and non-kin cooperation in humans. Here, we show for the first time that children cooperated less successfully with siblings than with non-kin children, whether or not non-kin partners were friends. Furthermore, children with larger social networks cooperated better and the perception of friendship among non-friends improved after cooperating. These results indicate that non-kin cooperation in humans has deep developmental foundations which might serve to forge and extend non-kin social relationships during middle childhood and create opportunities for future collaboration beyond kin. Our results provide a new framework for future studies focusing on how and why cooperation with different classes of partners may change during development in humans as well as other long-lived organisms.