This review article presents evidence for the claim that frequency effects are pervasive in children's first language acquisition, and hence constitute a phenomenon that any successful account must explain. The article is organized around four key domains of research: children's acquisition of single words, inflectional morphology, simple syntactic constructions, and more advanced constructions. In presenting this evidence, we develop five theses. (i) There exist different types of frequency effect, from effects at the level of concrete lexical strings to effects at the level of abstract cues to thematic-role assignment, as well as effects of both token and type, and absolute and relative, frequency. High-frequency forms are (ii) early acquired and (iii) prevent errors in contexts where they are the target, but also (iv) cause errors in contexts in which a competing lower-frequency form is the target. (v) Frequency effects interact with other factors (e.g. serial position, utterance length), and the patterning of these interactions is generally informative with regard to the nature of the learning mechanism. We conclude by arguing that any successful account of language acquisition, from whatever theoretical standpoint, must be frequency sensitive to the extent that it can explain the effects documented in this review, and outline some types of account that do and do not meet this criterion.

This article reports on an individual differences study that investigated the role of implicit statistical learning in the acquisition of syntax in children. One hundred children ages 4 years 5 months through 6 years 11 months completed a test of implicit statistical learning, a test of explicit declarative learning, and standardized tests of verbal and nonverbal ability. They also completed a syntactic priming task, which provided a dynamic index of children's facility to detect and respond to changes in the input frequency of linguistic structure. The results showed that implicit statistical learning ability was directly associated with the long-term maintenance of the primed structure. The results constitute the first empirical demonstration of a direct association between implicit statistical learning and syntactic acquisition in children.

We present the results from four studies, two corpora and two experimental, which suggest that English- and German-speaking children (3;1–4;9 years) use multiple constraints to process and produce object relative clauses. Our two corpora studies show that children produce object relatives that reflect the distributional and discourse regularities of the input. Specifically, the results show that when children produce object relatives they most often do so with (a) an inanimate head noun, and (b) a pronominal relative clause subject. Our experimental findings show that children use these constraints to process and produce this construction type. Moreover, when children were required to repeat the object relatives they most often use in naturalistic speech, the subject-object asymmetry in processing of relative clauses disappeared. We also report cross-linguistic differences in children's rate of acquisition which reflect properties of the input language. Overall, our results suggest that children are sensitive to the same constraints on relative clause processing as adults

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