Children expect prior belief strength to constrain belief revision

Across two preregistered studies (N = 199), we investigate whether children's developing theory of mind incorporates assumptions that people revise their beliefs rationally. Specifically, we focus on the role of prior belief strength (magnitude of and degree of certainty in the belief) in shaping children's judgments and predictions about others' belief revision. We presented four- to eight-year-old children with pairs of characters who hold the same belief, but differ in the strength and certainty of their belief. In Study 1, we found that children judge it possible to change both strong and weakly held beliefs-but by age five, children judge strongly held beliefs to be both less possible to change, and more difficult to change than weakly held ones. Additionally, children's open-ended explanations predominantly cited evidence as the reason for belief change. In Study 2, we tested whether children use prior belief strength to predict responses to counterevidence. After both characters encountered identical disconfirming testimony from a group of peers, children by age five predicted that characters with weak priors would revise their beliefs more readily than those with strong priors. In both studies, we found these effects of prior belief across diverse belief domains. Together, these studies offer evidence that children's developing theory of mind-in particular, their understanding of belief change-incorporates an assumption that beliefs are graded states whose revision is constrained by prior strength.