How human cognitive development relates to brain development remains poorly understood. For much of their history, developmental psychology and developmental neurobiology have been relatively independent of one another, owing in part to the widespread belief that cognitive development was explicable without reference to underlying substrates. This disciplinary isolation was predicated on a distinction between learning and maturation, whereby developmental psychologists studied processes of learning, which involved environmental interaction, while developmental neurobiologists studied processes of intrinsic maturation. The possibility that this cardinal distinction between learning and maturation might itself be misleading was rarely, if ever, considered. Hence, the possibility that cognitive processes of learning might play a role in regulating the growth of neural structures remains unexplored, both in terms of the degree of its empirical support and its implications for theories of cognitive development. Yet, this possibility holds a number of intriguing ramifications. If cognitive processes of learning do indeed play a role in regulating the growth of neural structures, then this would substantially alter the properties of what has come to be known as the “learning mechanism” in cognitive development (see Wexler & Culicover, 1980). In particular, it would demonstrate that the learning mechanism is nonstationary, meaning that its acquisition properties are time-dependent and change dynamically as a function of learning episodes. This would substantially alter how cognitive skills are acquired and would have to be taken into account in order to construct satisfactory theories of cognitive development. Because such a learning mechanism would introduce a number of time-dependent properties, which introduce many complications to the understanding of acquisition, there have been many attempts to exclude it on methodological grounds (Chomsky, 1980; Macnamara, 1982; Pinker, 1984). Primary among these attempts was Chomsky’s (1980) argument that language acquisition could be idealised as an instantaneous process without affecting its learning properties.