Preschoolers decide who is knowledgeable, who to inform, and who to trust via a causal understanding of how knowledge relates to action.

Preschoolers are discerning learners, preferring to trust people who are accurate, reliable, and appropriately-informed. Do these preferences reflect mental-state reasoning, where children infer what others know from their behavior, or do they reflect a reliance on simple cues? In Experiment 1 we show that four- and five-year-olds can infer knowledge from others' behavior when superficial cues and actions are matched across agents. Experiments 2a and 2b further suggest that children track how agents acquired their knowledge, and may use this to determine what different agents will (and will not) know. Finally, Experiment 3 shows that children independently make these knowledge inferences when deciding whom to trust. Our findings suggest that by age four, children have expectations about how knowledge relates to action, use these expectations to infer what others know from what they do, and rely on these inferences when deciding whom to trust.