An ability to integrate probability and epistemic reasoning is later-developing than either in isolation

When reasoning about others’ knowledge, intuitively we consider not just action outcomes like success or failure, but also the probability of such outcomes under different knowledge states. Across three experiments (n = 240 North American four- to six-year-olds) we find that even four-year-olds understand that tasks with a lower probability of random success are harder—but not until age six do children use this information to gauge (Experiment 1) and infer (Experiments 2-3) what others know. These results suggest that, although basic probabilistic reasoning and representations of knowledge are well in place by age four, children do not integrate the two to make mental-state inferences until much later, pointing to an area of important developmental change in Theory of Mind.