The properties of children's language are caused by those of their mothers', and vice-versa

Both the language used by young children (child language; CL) and the simplified language used by caretakers when talking to them (child-directed speech; CDS) become increasingly complex along development, eventually approaching regular adult language. Researchers disagree on whether children learn grammar from the input they receive (*usage-based theories*), or grammars are mostly innate, requiring only minimal input-based adjustments on the part of the children (*nativist theories*). A related question is whether parents adapt the complexity of CDS in specific response to their children's language abilities, or only in response to their level of general cognitive development. Parent-child interactions can be modelled by nonlinear dynamical systems. A technique recently developed in Ecology, Convergent Cross-Mapping (CCM), enables assessing causal relations between series from a nonlinear dynamical system. Here, we use CCM to reconstruct the network of *causal* relations between aspects of CL and CDS. This network supports a lexically-based syntax that is statistically learned. The results also indicate that mothers adapt CDS in response mainly to the *grammar* of CL. Our findings verify the strong causal predictions of usage-based theories, and are difficult for nativist theories to account for.