Abstract
Learning words that refer to mental states is a critical developmental milestone, yet how these words are learned remains poorly understood. In this study, 102 Chinese young children (Mage = 3.28; 45 girls) and 102 young adults (Mage = 22.67; 54 women) completed a novel word-learning task, in which a pseudoword was paired with videos depicting goal-directed reaching and distressed crying. Results indicated that children had difficulty learning mental state words, as reflected by their lower accuracy compared to adults. Diffusion model analyses further explored the underlying processes and found that children's difficulties did not reflect deficits in perceptual encoding of behavioral information or response execution. Instead, children accumulated evidence more slowly and adopted more conservative decision thresholds when inferring underlying mental states. Moreover, better theory-of-mind ability in children was associated with narrower decision thresholds, indicating that children with stronger social cognitive abilities required less evidence to learn mental-state meanings. Overall, these findings offer insights into the mechanisms underlying mental state word learning and advance our understanding of the complex interaction between children's social cognition and language development.