Word learning challenges explain nonadult possibility language comprehension in preschoolers.

This article presents two experiments testing English children's understanding of the "force" of modals, asking whether they understand that can expresses possibility and have_to expresses necessity. Prior studies show that children tend to over-accept necessity modals in possibility situations and argue this behavior stems from conceptual difficulties reasoning about open possibilities. However, these studies typically test modal force using epistemic modality (knowledge-based), which is less input-frequent than nonepistemic modalities (actual-world priorities or goals) and involves speaker perspective-taking. Our results with more familiar teleological (goal-oriented) modality show that preschoolers have an adult-like understanding of possibility can, but they seem to treat necessity have_to as a possibility modal, in affirmative (Experiment 1) and arguably in negative sentences (Experiment 2). We take these systematic errors to call into question conceptual accounts. We argue that younger preschoolers' difficulties with modal force are due to word-learning challenges: They treat necessity modals as possibility modals. (PsycInfo Database Record (c) 2025 APA, all rights reserved).