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I first outline three major developments in child language research over the past forty years: the use of computational modelling to reveal the structure of information in the input; the focus on quantifying productivity and abstraction; and developments in the explanation of systematic errors. Next, I turn to what I consider to be major outstanding issues: how the network of constructions builds up and the relationship between social and cognitive development and language learning. Finally, I briefly consider a number of other areas of importance to a psychologically realistic understanding of children's language development.
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Elena Lieven (2014) studied this question.
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