Child-Directed Speech Is Semantically Constrained: Evidence From Metaphoricity and Concreteness

Child-directed speech (CDS) is widely theorized to provide a structured scaffold for early language acquisition, yet the semantic properties of this register remain insufficiently quantified. In particular, it remains unclear how figurative language and semantic grounding interact within caregiver input to stabilize early form-meaning mappings. We present a large-scale computational analysis of metaphoricity and lexical concreteness in a naturalistic corpus of CDS, evaluated against a size-matched baseline of adult-directed speech (ADS). Using a token-level metaphor detection model and human-indexed semantic norms, we test two primary accounts: the reduced metaphoricity hypothesis (H1), which predicts a systematic reduction of nonliteral expressions in caregiver input, and the lexical concreteness hypothesis (H2), which predicts an intensified reliance on perceptually grounded vocabulary. Cross-register comparisons revealed robust support for both accounts: CDS contained a significantly lower density of metaphorical expressions and a higher baseline of both continuous and categorical lexical concreteness than ADS. Longitudinal mixed-effects models further showed that these semantic constraints undergo gradual, age-dependent tuning. As children mature, caregivers appear to relax these constraints incrementally, diversifying their figurative repertoire and reducing overall vocabulary concreteness over time. Taken together, these findings suggest that the semantic structure of caregiver speech functions as a dynamic, responsive learning environment. By initially favoring concrete, transparent, and literal forms, CDS may reduce interpretive ambiguity during early phases of reference anchoring before gradually shifting toward the more abstract and flexible lexical patterns characteristic of adult speech.