When children can explain why they believe a claim, they suggest a better empirical test for that claim.

We tested the hypothesis that children's ability to reflect on the causes of their uncertainty about a surprising claim allows them to better target their empirical investigation of that claim-and that this ability increases with age. We assigned 4-7-year-old children (=174, M= 68.77 months, 52.87% girls) to either a prompted or an unprompted condition. In each condition, children witnessed a series of vignettes where an adult presented a surprising claim about an object. Children were then asked whether they thought the claim was true or not, how certain or uncertain they were, and how they would test that claim. In the prompted condition, children were also asked why they were certain or uncertain. As predicted, older children were more likely to justify their beliefs and to suggest targeted empirical tests, compared with younger children. Being prompted to reflect on their uncertainty did not increase children's ability to an efficient test for those claims. However, exploratory analyses revealed that children's ability to provide a plausible reason for their beliefs did, controlling for their ability to an efficient test for a claim. This suggests that developments in children's reasoning about their beliefs allow them to more effectively assess those beliefs empirically.