The way infants learn language is a highly complex adaptive behavior. This behavior chiefly relies on the ability to extract information from the speech they hear and combine it with information from the external environment. Most theories assume that this ability critically hinges on the recognition of at least some syntactic structure. Here, we show that child-directed speech allows for semantic inference without relying on explicit structural information. We simulate the process of semantic inference with machine learning applied to large text collections of two different types of speech, child-directed speech versus adult-directed speech. Taking the core meaning of causality as a test case, we find that in child-directed speech causal meaning can be successfully inferred from simple co-occurrences of neighboring words. By contrast, semantic inference in adult-directed speech fundamentally requires additional access to syntactic structure. These results suggest that child-directed speech is ideally shaped for a learner who has not yet mastered syntactic structure.

Children acquire their first language while interacting with adults in a highly adaptive manner. While adaptation occurs at many linguistic levels such as syntax and speech complexity, semantic adaptation remains unclear due to the difficulty of efficient meaning extraction. In this study, we examine the adaptation of semantics with a computational approach based on distributional information. We show that adults, in their speech addressed to children, adapt their distributional semantics to that in the speech children produce. By analyzing semantic representations modeled from the Manchester corpus, a large longitudinal acquisition corpus of English, we find striking similarity of semantic development between child and child-directed speech, with a slight time lag in the latter. These findings provide strong evidence for the semantic adaptation in first language acquisition and suggest the important role of child-directed speech in semantic learning.

Children’s spontaneous speech may not reflect true productivity with grammar rules. We investigated Turkish-learning children’s rule-based understanding of causative morphology by combining experimental and corpus work. We asked (1) when the generalization of causative morphology emerges and (2) what role input plays in this development. To answer the first question, Study 1 experimentally tested 106 children aged 2;6–6;1 on a language judgment task using pseudo-verbs. Children preferred the causative marker -DIr over an incongruent suffix for causativized events earliest at age 4;10. Further, Study 2 tested 38 children aged 3;6–4;6 and revealed no preference of -DIr over no suffix. Study 3 examined a corpus for child-directed input and showed that the variation of verb stems to which -DIr was attached remained lower than variation of verbs eligible to take -DIr, up to age 3. The findings suggest that rule-based understanding of morphological causatives emerges much later than previously proposed productivity at age 2, which might be accounted for by the insufficient variation of morphological causatives in the early input.

Becoming productive with grammatical categories is a gradual process in children's language development. Here, we investigated this transition process by focusing on Turkish causatives. Previous research examining spontaneous and elicited production of Turkish causatives with familiar verbs attested the onset and early stages of productivity at ages 2 to 3 (Aksu‐Koç & Slobin, 1985; Nakipoğlu, Uzundag, & Sarıgül, 2021). So far, however, we know very little about children's understanding of causatives with novel verbs. In the present study, we asked: (a) When does the generalization of causative morphology in a novel context emerge? and (b) What role does child‐directed input play in this development? To answer the first question, we conducted comprehension‐judgment experiments with children aged 2;6–6;11 using pseudo‐verbs (Study 1 & 2). Results showed that children preferred the Turkish causative suffix ‐DIr over an unrelated or no suffix to denote caused events earliest at age 4;10. To answer the second question, we analyzed child‐directed speech from a longitudinal corpus of Turkish language acquisition (Study 3). Results showed that when addressing children younger than age 3, caregivers used the ‐DIr suffix with little variation considering the overall variability of verbs they could utter. Overall, these findings suggest that productivity with morphological causatives in a novel context emerges in a later stage of acquisition. This later development might partly be accounted for by the insufficient variation of morphological causatives in the early input.