If they must, they will: Children overcommit to likeliness inferences from deontic modals

Modal verbs like must express two distinct non-actual meanings: deontic (e.g., obligation) and epistemic (e.g., inference). How do young children understand these modals? What factors affect their interpretation as deontic or epistemic? We report a picture preference task testing preschool children’s interpretations of Bosnian/Croatian/Serbian (BCS) morati ‘must’ as deontic or epistemic. Prior work on English must shows that despite an early deontic comprehension bias at age 3, by age 5 children have flipped to a strong epistemic bias, including for constructions adults prefer deontic interpretations (must + eventive verbs). However, properties of English leave open multiple explanations for this non-adult behaviour, as must is primarily epistemic in the input, and must + eventive verb constructions can also receive epistemic interpretations. BCS morati provides a natural comparison: morati is overwhelmingly deontic in the input, and BCS syntax provides categorical cues to deontic versus epistemic interpretation. Our results show that BCS children are more adult-like at age 3 than English children, a difference we attribute to clearer syntactic cues to flavour in BCS. But, by age 5 BCS children behave like English counterparts, preferring epistemic interpretations even for constructions that are deontic-only in BCS. We argue this cross-linguistic result is best explained pragmatically: deontic uses of both morati and must invite a likelihood inference that obligations will be normatively carried out. This inference was first proposed to explain diachronic meaning changes from root-to-epistemic. We show older preschool children commit to this likelihood inference more than adults.