From Galileo to Gandhi, and from Plato to Piaget, influential thinkers throughout history have highlighted the benefits of disagreement for science, society, and the individual. Despite the rich theoretical interest, the specific individual benefits of disagreement have often remained unclear. To address this gap, the current dissertation explores one particular individual psychological consequence of disagreement: how it prompts metacognitive reflection during early childhood. The ability to metacognitively reflect on one’s own knowledge plays a critical role in learning, as well as in individual and joint decision-making. Yet, young children’s metacognitive capacities are often still limited in significant ways. By middle childhood, however, children’s metacognitive competence has significantly improved. What explains this striking change? This dissertation argues that metacognitive development is centrally driven by young children’s social experiences of disagreement.

To begin to test this hypothesis, the effects of disagreement on young children’s metacognition are explored across three chapters, each focusing on a distinct, frequently studied dimension of metacognition: reason-giving, confidence ratings, and rational belief revision. Chapter 2 uses a cross-cultural approach, finding that experiencing disagreement (more so than agreement) leads children from three diverse cultural backgrounds to reflect on their reasons for their beliefs when making joint decisions. Chapter 3 demonstrates that disagreeing (versus agreeing) with another individual reduces young children's overconfidence and increases their motivation to search for the correct answer. Finally, Chapter 4 finds that children flexibly revise their initial beliefs, or suspend judgment until they have acquired additional evidence, depending on the strength of the evidence supporting their own belief versus that of a disagreeing other.

Together, these findings provide clear evidence that experiencing disagreement prompts young children’s metacognitive reflection. Theoretically, these insights are significant as they bridge the gap between prior theoretical perspectives that have emphasized the role of social interaction in cognitive development, such as constructivist learning theories and cultural evolutionary accounts of metacognition, refines them, and makes them empirically testable. Moreover, the current work has important practical implications, and could inform interventions aimed at fostering learning and reasoning, as well as promoting mutual understanding.