3-year-olds engage in joint action, but 5-year-olds engage in joint ideation.

Collaboration is a foundational element of human social behavior. This can mean joint action, such as hunting deer together, or building a barn together. But it can also mean 'joint ideation', where we collaborate on the creation of plans, beliefs, or theories - we may collaborate on a new plan for community governance, or on a theory about how planets orbit the sun. Here we compare three- and five-year olds' performance on collaborative tasks that required either joint action, or joint ideation. First, children had to play a game that required coordinated physical action while they could watch one another. This could be called 'behaviorally mediated' joint action, and is the typical focus of studies of collaboration. In a second task, children had to agree together which game they would play, before separately retrieving appropriate parts for that game. This required the children to come up with a joint plan before acting, and follow through on the plan without being able to observe one another's actions. We found that three- and five-year-olds engaged in joint action, but only five-year-olds succeeded in creating a joint plan before acting. We found that performance in explicit false belief tasks predicted joint ideation, indicating the centrality of advanced theory of mind to this later developing ability. These results add to our understanding of the nature and development of human collaboration, showing that collaboration on ideas, but not actions, depends on mind-reading skills not yet available to the youngest children.